Libguestfs - Jan 2019 - Re: [PATCH nbdkit v2 3/4] cache: Implement LRU structure.

These are essentially identical to what was previously posted as
patches 6/9 through 9/9 here:

  https://www.redhat.com/archives/libguestfs/2018-December/msg00145.html

except that it has been rebased onto the current git master and
retested thoroughly.

Rich.

It's useful to be able to search for the next non-zero entry in a
bitmap.  This commit adds a ?bitmap_next? function to do that.
Because the bitmap is just a uint8_t buffer, using fast string
functions we should be able to do this quickly even if the bitmap is
sparse.  (However the actual implementation is not optimized since
that is quite complicated - see to-do comments in
common/include/nextnonzero.h).

I wasn't confident about the correctness of the code and so this
commit also adds some unit tests covering all of the bitmap code.
---
 common/bitmap/bitmap.h       |   6 ++
 common/include/iszero.h      |   3 +
 common/include/nextnonzero.h |  59 +++++++++++++
 common/bitmap/bitmap.c       |  35 ++++++++
 common/bitmap/test-bitmap.c  | 162 +++++++++++++++++++++++++++++++++++
 .gitignore                   |   1 +
 common/bitmap/Makefile.am    |  12 +++
 common/include/Makefile.am   |   1 +
 8 files changed, 279 insertions(+)

diff --git a/common/bitmap/bitmap.h b/common/bitmap/bitmap.h
index f943933..80fd5e4 100644
--- a/common/bitmap/bitmap.h
+++ b/common/bitmap/bitmap.h
@@ -165,4 +165,10 @@ bitmap_set (const struct bitmap *bm, uint64_t offset,
unsigned v)
 #define bitmap_for(bm, /* uint64_t */ blknum)                           \
   for ((blknum) = 0; (blknum) < (bm)->size * (bm)->ibpb; ++(blknum))
 
+/* Find the next non-zero block in the bitmap, starting at ?blk?.
+ * Returns -1 if the bitmap is all zeroes from blk to the end of the
+ * bitmap.
+ */
+extern int64_t bitmap_next (const struct bitmap *bm, uint64_t blk);
+
 #endif /* NBDKIT_BITMAP_H */
diff --git a/common/include/iszero.h b/common/include/iszero.h
index 1079f3e..7a54f0a 100644
--- a/common/include/iszero.h
+++ b/common/include/iszero.h
@@ -42,6 +42,9 @@
  * The clever approach here was suggested by Eric Blake.  See:
  * https://www.redhat.com/archives/libguestfs/2017-April/msg00171.html
  * https://rusty.ozlabs.org/?p=560
+ *
+ * See also:
+ * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69908
  */
 static inline bool
 is_zero (const char *buffer, size_t size)
diff --git a/common/include/nextnonzero.h b/common/include/nextnonzero.h
new file mode 100644
index 0000000..3f96e85
--- /dev/null
+++ b/common/include/nextnonzero.h
@@ -0,0 +1,59 @@
+/* nbdkit
+ * Copyright (C) 2018 Red Hat Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef NBDKIT_NEXTNONZERO_H
+#define NBDKIT_NEXTNONZERO_H
+
+/* Given a byte buffer, return a pointer to the first non-zero byte,
+ * or return NULL if we reach the end of the buffer.
+ *
+ * XXX: Even with -O3 -mavx2, gcc 8.2.1 makes a terrible job with this
+ * loop, compiling it completely naively.  QEMU has an AVX2
+ * implementation of a similar loop.
+ *
+ * See also:
+ * https://sourceware.org/bugzilla/show_bug.cgi?id=19920
+ * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69908
+ */
+static inline const char *
+next_non_zero (const char *buffer, size_t size)
+{
+  size_t i;
+
+  for (i = 0; i < size; ++i)
+    if (buffer[i] != 0)
+      return &buffer[i];
+  return NULL;
+}
+
+#endif /* NBDKIT_NEXTNONZERO_H */
diff --git a/common/bitmap/bitmap.c b/common/bitmap/bitmap.c
index fb5dbe7..9690a2e 100644
--- a/common/bitmap/bitmap.c
+++ b/common/bitmap/bitmap.c
@@ -42,6 +42,7 @@
 
 #include "bitmap.h"
 #include "rounding.h"
+#include "nextnonzero.h"
 
 int
 bitmap_resize (struct bitmap *bm, uint64_t new_size)
@@ -73,3 +74,37 @@ bitmap_resize (struct bitmap *bm, uint64_t new_size)
 
   return 0;
 }
+
+int64_t
+bitmap_next (const struct bitmap *bm, uint64_t blk)
+{
+  uint64_t limit = bm->size * bm->ibpb;
+  const uint8_t *p;
+
+  /* Align to the next byte boundary. */
+  for (; blk < limit && (blk & (bm->ibpb-1)) != 0; ++blk) {
+    if (bitmap_get_blk (bm, blk, 0) != 0)
+      return blk;
+  }
+  if (blk == limit)
+    return -1;
+
+  /* Now we're at a byte boundary we can use a fast string function to
+   * find the next non-zero byte.
+   */
+  p = &bm->bitmap[blk >> (3 - bm->bitshift)];
+  p = (const uint8_t *) next_non_zero ((const char *) p,
+                                       &bm->bitmap[bm->size] - p);
+  if (p == NULL)
+    return -1;
+
+  /* Now check the non-zero byte to find out which bit (ie. block) is set. */
+  blk = (p - bm->bitmap) << (3 - bm->bitshift);
+  for (; blk < limit; ++blk) {
+    if (bitmap_get_blk (bm, blk, 0) != 0)
+      return blk;
+  }
+
+  /* Should never be reached. */
+  abort ();
+}
diff --git a/common/bitmap/test-bitmap.c b/common/bitmap/test-bitmap.c
new file mode 100644
index 0000000..6bf4574
--- /dev/null
+++ b/common/bitmap/test-bitmap.c
@@ -0,0 +1,162 @@
+/* nbdkit
+ * Copyright (C) 2018 Red Hat Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/* Unit tests of the bitmap code. */
+
+#include <config.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdarg.h>
+#include <string.h>
+#include <errno.h>
+#include <assert.h>
+
+#include <nbdkit-plugin.h>
+
+#include "bitmap.h"
+
+static void
+test (int bpb, int blksize)
+{
+  struct bitmap bm;
+  const int nr_blocks = 1000;
+  int blks[] +    {
+     1, 2, 3, 10, 12,
+     90, 91, 92, 93, 94, 99,
+     800, 801, 802, 803,
+     902, 903, 905, 907, 911, 913, 917, 919, 923, 929,
+     999
+    };
+  unsigned v, vexp;
+  size_t i, j;
+
+  printf ("bpb = %d, blksize = %d\n", bpb, blksize);
+  fflush (stdout);
+
+  bitmap_init (&bm, blksize, bpb);
+  if (bitmap_resize (&bm, nr_blocks * blksize) == -1)
+    exit (EXIT_FAILURE);
+
+  /* Set some bits at known block numbers. */
+  for (j = 0; j < sizeof blks / sizeof blks[0]; ++j) {
+    v = (j & 1) == 0 ? 1 : (1<<bpb) - 1;
+    bitmap_set_blk (&bm, blks[j], v);
+  }
+
+  /* Check the values of all bits. */
+  for (i = j = 0; i < nr_blocks; ++i) {
+    if (i == blks[j]) { /* previously set bit */
+      vexp = (j & 1) == 0 ? 1 : (1<<bpb) - 1;
+      v = bitmap_get_blk (&bm, blks[j], 0);
+      assert (v == vexp);
+      ++j;
+    }
+    else { /* unset bit, except it to be zero */
+      v = bitmap_get_blk (&bm, i, 0);
+      assert (v == 0);
+    }
+  }
+
+  /* Same as above, but using bitmap_for macro. */
+  j = 0;
+  bitmap_for (&bm, i) {
+    if (i == blks[j]) { /* previously set bit */
+      vexp = (j & 1) == 0 ? 1 : (1<<bpb) - 1;
+      v = bitmap_get_blk (&bm, blks[j], 0);
+      assert (v == vexp);
+      ++j;
+    }
+    else { /* unset bit, except it to be zero */
+      v = bitmap_get_blk (&bm, i, 0);
+      assert (v == 0);
+    }
+  }
+
+  /* Use bitmap_next to iterate over the non-zero entries in the bitmap. */
+  i = bitmap_next (&bm, 0);
+  j = 0;
+  while (i != -1) {
+    assert (i == blks[j]);
+    vexp = (j & 1) == 0 ? 1 : (1<<bpb) - 1;
+    v = bitmap_get_blk (&bm, i, 0);
+    assert (v == vexp);
+    i = bitmap_next (&bm, i+1);
+    ++j;
+  }
+
+  bitmap_free (&bm);
+}
+
+int
+main (void)
+{
+  int bpb;
+  size_t i;
+  int blksizes[] = { 1, 2, 4, 1024, 2048, 4096, 16384 };
+
+  /* Try the tests at each bpb setting and at a range of block sizes. */
+  for (bpb = 1; bpb <= 8; bpb <<= 1)
+    for (i = 0; i < sizeof blksizes / sizeof blksizes[0]; ++i)
+      test (bpb, blksizes[i]);
+
+  exit (EXIT_SUCCESS);
+}
+
+/* The bitmap code uses nbdkit_debug, normally provided by the main
+ * server program.  So we have to provide it here.
+ */
+void
+nbdkit_debug (const char *fs, ...)
+{
+  /* do nothing */
+}
+
+/* Same for nbdkit_error. */
+void
+nbdkit_error (const char *fs, ...)
+{
+  int err = errno;
+  va_list args;
+
+  va_start (args, fs);
+  fprintf (stderr, "error: ");
+  errno = err; /* Must restore in case fs contains %m */
+  vfprintf (stderr, fs, args);
+  fprintf (stderr, "\n");
+  va_end (args);
+
+  errno = err;
+}
diff --git a/.gitignore b/.gitignore
index e5b5d82..fc019e9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -24,6 +24,7 @@ Makefile.in
 
 /aclocal.m4
 /autom4te.cache
+/common/bitmap/test-bitmap
 /common/include/test-byte-swapping
 /common/include/test-current-dir-name
 /common/include/test-isaligned
diff --git a/common/bitmap/Makefile.am b/common/bitmap/Makefile.am
index cbd82bd..dade5ec 100644
--- a/common/bitmap/Makefile.am
+++ b/common/bitmap/Makefile.am
@@ -42,3 +42,15 @@ libbitmap_la_CPPFLAGS = \
 	-I$(top_srcdir)/common/include
 libbitmap_la_CFLAGS = \
         $(WARNINGS_CFLAGS)
+
+# Unit tests.
+
+TESTS = test-bitmap
+check_PROGRAMS = test-bitmap
+
+test_bitmap_SOURCES = test-bitmap.c bitmap.c bitmap.h
+test_bitmap_CPPFLAGS = \
+	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/include
+test_bitmap_CFLAGS = \
+        $(WARNINGS_CFLAGS)
diff --git a/common/include/Makefile.am b/common/include/Makefile.am
index 506fe2c..3e7f056 100644
--- a/common/include/Makefile.am
+++ b/common/include/Makefile.am
@@ -41,6 +41,7 @@ EXTRA_DIST = \
 	isaligned.h \
 	ispowerof2.h \
 	iszero.h \
+	nextnonzero.h \
 	random.h \
 	rounding.h
 
-- 
2.19.2

Clears the bitmap.
---
 common/bitmap/bitmap.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/common/bitmap/bitmap.h b/common/bitmap/bitmap.h
index 80fd5e4..0da650a 100644
--- a/common/bitmap/bitmap.h
+++ b/common/bitmap/bitmap.h
@@ -42,6 +42,7 @@
 #define NBDKIT_BITMAP_H
 
 #include <stdint.h>
+#include <string.h>
 #include <assert.h>
 
 #include <nbdkit-plugin.h>
@@ -100,6 +101,13 @@ bitmap_free (struct bitmap *bm)
  */
 extern int bitmap_resize (struct bitmap *bm, uint64_t new_size);
 
+/* Clear the bitmap (set everything to zero). */
+static inline void
+bitmap_clear (struct bitmap *bm)
+{
+  memset (bm->bitmap, 0, bm->size);
+}
+
 /* This macro calculates the byte offset in the bitmap and which
  * bit/mask we are addressing within that byte.
  *
-- 
2.19.2

---
 filters/cache/lru.h       |  54 ++++++++++++++
 filters/cache/blk.c       |  12 +++
 filters/cache/lru.c       | 150 ++++++++++++++++++++++++++++++++++++++
 filters/cache/Makefile.am |   2 +
 4 files changed, 218 insertions(+)

diff --git a/filters/cache/lru.h b/filters/cache/lru.h
new file mode 100644
index 0000000..4faefcd
--- /dev/null
+++ b/filters/cache/lru.h
@@ -0,0 +1,54 @@
+/* nbdkit
+ * Copyright (C) 2018 Red Hat Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef NBDKIT_LRU_H
+#define NBDKIT_LRU_H
+
+#include <stdbool.h>
+
+/* Initialize LRU. */
+extern void lru_init (void);
+
+/* Free the LRU. */
+extern void lru_free (void);
+
+/* Notify LRU that the virtual size has changed. */
+extern int lru_set_size (uint64_t new_size);
+
+/* Mark a block as recently accessed in the LRU structure. */
+extern void lru_set_recently_accessed (uint64_t blknum);
+
+/* Check if a block has been recently accessed. */
+extern bool lru_has_been_recently_accessed (uint64_t blknum);
+
+#endif /* NBDKIT_LRU_H */
diff --git a/filters/cache/blk.c b/filters/cache/blk.c
index 4000276..b256446 100644
--- a/filters/cache/blk.c
+++ b/filters/cache/blk.c
@@ -53,6 +53,7 @@
 
 #include "cache.h"
 #include "blk.h"
+#include "lru.h"
 
 /* The cache. */
 static int fd = -1;
@@ -80,6 +81,8 @@ blk_init (void)
   size_t len;
   char *template;
 
+  lru_init ();
+
   bitmap_init (&bm, BLKSIZE, 2 /* bits per block */);
 
   tmpdir = getenv ("TMPDIR");
@@ -115,6 +118,8 @@ blk_free (void)
     close (fd);
 
   bitmap_free (&bm);
+
+  lru_free ();
 }
 
 int
@@ -128,6 +133,9 @@ blk_set_size (uint64_t new_size)
     return -1;
   }
 
+  if (lru_set_size (new_size) == -1)
+    return -1;
+
   return 0;
 }
 
@@ -163,6 +171,7 @@ blk_read (struct nbdkit_next_ops *next_ops, void *nxdata,
         return -1;
       }
       bitmap_set_blk (&bm, blknum, BLOCK_CLEAN);
+      lru_set_recently_accessed (blknum);
     }
     return 0;
   }
@@ -172,6 +181,7 @@ blk_read (struct nbdkit_next_ops *next_ops, void *nxdata,
       nbdkit_error ("pread: %m");
       return -1;
     }
+    lru_set_recently_accessed (blknum);
     return 0;
   }
 }
@@ -196,6 +206,7 @@ blk_writethrough (struct nbdkit_next_ops *next_ops, void
*nxdata,
     return -1;
 
   bitmap_set_blk (&bm, blknum, BLOCK_CLEAN);
+  lru_set_recently_accessed (blknum);
 
   return 0;
 }
@@ -222,6 +233,7 @@ blk_write (struct nbdkit_next_ops *next_ops, void *nxdata,
     return -1;
   }
   bitmap_set_blk (&bm, blknum, BLOCK_DIRTY);
+  lru_set_recently_accessed (blknum);
 
   return 0;
 }
diff --git a/filters/cache/lru.c b/filters/cache/lru.c
new file mode 100644
index 0000000..60cf379
--- /dev/null
+++ b/filters/cache/lru.c
@@ -0,0 +1,150 @@
+/* nbdkit
+ * Copyright (C) 2018 Red Hat Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/* These are the block operations.  They always read or write a single
+ * whole block of size ?blksize?.
+ */
+
+#include <config.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include <inttypes.h>
+
+#include <nbdkit-filter.h>
+
+#include "bitmap.h"
+
+#include "cache.h"
+#include "blk.h"
+#include "lru.h"
+
+#define MAX(a, b) ((a) > (b) ? (a) : (b))
+
+/* LRU bitmaps.  These bitmaps implement a simple, fast LRU structure.
+ *
+ *    bm[0]         bm[1]         blocks not in either bitmap
+ * ????????????????????????????????????????????????????????????
+ * ?         ?                  ?                             ?
+ * ????????????????????????????????????????????????????????????
+ *    ?          c1 bits set
+ *  c0 bits set
+ *
+ * The LRU structure keeps track of the [approx] last N distinct
+ * blocks which have been most recently accessed.  It can answer in
+ * O(1) time the question: ?Is a particular block in or not in the N
+ * distinct blocks most recently accessed??
+ *
+ * To do this we keep two bitmaps.
+ *
+ * When a block is accessed, we set the corresponding bit in bm[0] and
+ * increment c0 (c0 counts the number of bits set in bm[0]).  If c0 =+ * N/2
then we swap the two bitmaps, clear bm[0], and reset c0 to 0.
+ *
+ * To check if a block has been accessed within the previous N
+ * distinct accesses, we simply have to check both bitmaps.  If it is
+ * not in either bitmap, then it's old and a candidate to be
+ * reclaimed.
+ *
+ * You'll note that in fact we only keep track of between N/2 and N
+ * recently accessed blocks.  We could make the estimate more accurate
+ * by having more bitmaps, but as this is only a heuristic we choose
+ * to keep the implementation simple and memory usage low instead.
+ */
+static struct bitmap bm[2];
+static unsigned c0 = 0, c1 = 0;
+static unsigned N = 100;
+
+void
+lru_init (void)
+{
+  bitmap_init (&bm[0], BLKSIZE, 1 /* bits per block */);
+  bitmap_init (&bm[1], BLKSIZE, 1 /* bits per block */);
+}
+
+void
+lru_free (void)
+{
+  bitmap_free (&bm[0]);
+  bitmap_free (&bm[1]);
+}
+
+int
+lru_set_size (uint64_t new_size)
+{
+  if (bitmap_resize (&bm[0], new_size) == -1)
+    return -1;
+  if (bitmap_resize (&bm[1], new_size) == -1)
+    return -1;
+
+  /* XXX Choose this better. */
+  N = MAX (new_size / BLKSIZE / 4, 100);
+
+  return 0;
+}
+
+void
+lru_set_recently_accessed (uint64_t blknum)
+{
+  /* If the block is already set in the first bitmap, don't need to do
+   * anything.
+   */
+  if (bitmap_get_blk (&bm[0], blknum, false))
+    return;
+
+  bitmap_set_blk (&bm[0], blknum, true);
+  c0++;
+
+  /* If we've reached N/2 then we need to swap over the bitmaps. */
+  if (c0 >= N/2) {
+    struct bitmap tmp;
+
+    tmp = bm[0];
+    bm[0] = bm[1];
+    bm[1] = tmp;
+    c1 = c0;
+
+    bitmap_clear (&bm[0]);
+    c0 = 0;
+  }
+}
+
+bool
+lru_has_been_recently_accessed (uint64_t blknum)
+{
+  return
+    bitmap_get_blk (&bm[0], blknum, false) ||
+    bitmap_get_blk (&bm[1], blknum, false);
+}
diff --git a/filters/cache/Makefile.am b/filters/cache/Makefile.am
index 3a542af..1620fa1 100644
--- a/filters/cache/Makefile.am
+++ b/filters/cache/Makefile.am
@@ -41,6 +41,8 @@ nbdkit_cache_filter_la_SOURCES = \
 	blk.h \
 	cache.c \
 	cache.h \
+	lru.c \
+	lru.h \
 	$(top_srcdir)/include/nbdkit-filter.h
 
 nbdkit_cache_filter_la_CPPFLAGS = \
-- 
2.19.2

The original plan was to have a background thread doing the reclaim.
However that cannot work given the design of filters, because a
background thread cannot access the next_ops struct which is only
available during requests.

Therefore we spread the work over the request threads.  Each blk_*
function checks whether there is work to do, and if there is will
reclaim up to two blocks from the cache (thus ensuring that we always
make forward progress, since each blk_* function can only add at most
one block to the cache).
---
 filters/cache/nbdkit-cache-filter.pod |  71 +++++++++++
 filters/cache/cache.h                 |  13 ++
 filters/cache/blk.c                   | 163 ++++++++++++++++++++++++++
 filters/cache/cache.c                 |  56 +++++++++
 filters/cache/lru.c                   |   7 +-
 TODO                                  |  11 +-
 tests/Makefile.am                     |   4 +-
 tests/test-cache-max-size.sh          |  96 +++++++++++++++
 8 files changed, 409 insertions(+), 12 deletions(-)

diff --git a/filters/cache/nbdkit-cache-filter.pod
b/filters/cache/nbdkit-cache-filter.pod
index 09dc39f..1280959 100644
--- a/filters/cache/nbdkit-cache-filter.pod
+++ b/filters/cache/nbdkit-cache-filter.pod
@@ -5,6 +5,9 @@ nbdkit-cache-filter - nbdkit caching filter
 =head1 SYNOPSIS
 
  nbdkit --filter=cache plugin [cache=writeback|writethrough|unsafe]
+                              [cache-max-size=SIZE]
+                              [cache-high-threshold=N]
+                              [cache-low-threshold=N]
                               [cache-on-read=true|false]
                               [plugin-args...]
 
@@ -51,6 +54,14 @@ This is dangerous and can cause data loss, but this may be
acceptable
 if you only use it for testing or with data that you don't care about
 or can cheaply reconstruct.
 
+=item B<cache-max-size=SIZE>
+
+=item B<cache-high-threshold=N>
+
+=item B<cache-low-threshold=N>
+
+Limit the size of the cache to C<SIZE>.  See L</CACHE MAXIMUM SIZE>
below.
+
 =item B<cache-on-read=true>
 
 Cache read requests as well as write requests.  Any time a block is
@@ -69,6 +80,66 @@ Do not cache read requests (this is the default).
 
 =back
 
+=head1 CACHE MAXIMUM SIZE
+
+By default the cache can grow to any size (although not larger than
+the virtual size of the underlying plugin) and you have to ensure
+there is sufficient space in C<$TMPDIR> for it.
+
+Using the parameters C<cache-max-size>, C<cache-high-threshold> and
+C<cache-low-threshold> you can limit the maximum size of the cache.
+
+Some examples:
+
+=over 4
+
+=item C<cache-max-size=1G>
+
+The cache is limited to around 1 gigabyte.
+
+=item C<cache-max-size=1G cache-high-threshold=95 cache-low-threshold=80>
+
+Once the cache size reaches 972M (95% of 1G), blocks are reclaimed
+from the cache until the size is reduced to 819M (80% of 1G).
+
+=back
+
+The way this works is once the size of the cache exceeds
+S<C<SIZE> ? the high threshold>, the filter works to reduce the
size
+of the cache until it is less than S<C<SIZE> ? the low threshold>.
+Once the size is below the low threshold, no more reclaim work is done
+until the size exceeds the high threshold again.
+
+The default thresholds are high 95% and low 80%.  The thresholds are
+expressed as percentages of C<cache-max-size>, and may be greater than
+100.
+
+The reclaiming work discards blocks from the cache in the following
+order of priority:
+
+=over 4
+
+=item *
+
+(Highest priority / first to be reclaimed.)
+Least recently used clean blocks are discarded from the cache.
+
+=item *
+
+Least recently used dirty blocks are flushed to the plugin and
+discarded.
+
+=item *
+
+Recently used clean blocks are discarded.
+
+=item *
+
+(Lowest priority / last to be reclaimed.)
+Recently used dirty blocks are flushed to the plugin and discarded.
+
+=back
+
 =head1 ENVIRONMENT VARIABLES
 
 =over 4
diff --git a/filters/cache/cache.h b/filters/cache/cache.h
index 50416b1..233a4fa 100644
--- a/filters/cache/cache.h
+++ b/filters/cache/cache.h
@@ -34,6 +34,8 @@
 #ifndef NBDKIT_CACHE_H
 #define NBDKIT_CACHE_H
 
+#include <fcntl.h>
+
 /* Size of a block in the cache.  A 4K block size means that we need
  * 64 MB of memory to store the bitmaps for a 1 TB underlying image.
  * It is also smaller than the usual hole size for sparse files, which
@@ -48,7 +50,18 @@ extern enum cache_mode {
   CACHE_MODE_UNSAFE,
 } cache_mode;
 
+/* Maximum size of the cache and high/low thresholds. */
+extern int64_t max_size;
+extern int hi_thresh, lo_thresh;
+
 /* Cache read requests. */
 extern bool cache_on_read;
 
+/* Do we support reclaiming cache blocks? */
+#ifdef FALLOC_FL_PUNCH_HOLE
+#define HAVE_CACHE_RECLAIM 1
+#else
+#undef HAVE_CACHE_RECLAIM
+#endif
+
 #endif /* NBDKIT_CACHE_H */
diff --git a/filters/cache/blk.c b/filters/cache/blk.c
index b256446..6f67a7f 100644
--- a/filters/cache/blk.c
+++ b/filters/cache/blk.c
@@ -46,6 +46,8 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <sys/types.h>
+#include <sys/stat.h>
 
 #include <nbdkit-filter.h>
 
@@ -74,6 +76,31 @@ enum bm_entry {
   BLOCK_DIRTY = 3,
 };
 
+#ifdef HAVE_CACHE_RECLAIM
+/* If we are currently reclaiming blocks from the cache.
+ *
+ * The state machine starts in the NOT_RECLAIMING state.  When the
+ * size of the cache exceeds the high threshold, we move to
+ * RECLAIMING_LRU.  Once we have exhausted all LRU blocks, we move to
+ * RECLAIMING_ANY (reclaiming any blocks).
+ *
+ * If at any time the size of the cache goes below the low threshold
+ * we move back to the NOT_RECLAIMING state.
+ *
+ * reclaim_blk is the last block that we will looked at.
+ */
+enum reclaim_state {
+  NOT_RECLAIMING = 0,
+  RECLAIMING_LRU = 1,
+  RECLAIMING_ANY = 2,
+};
+
+static enum reclaim_state reclaiming = NOT_RECLAIMING;
+static uint64_t reclaim_blk;
+#endif
+
+static void reclaim (void);
+
 int
 blk_init (void)
 {
@@ -146,6 +173,8 @@ blk_read (struct nbdkit_next_ops *next_ops, void *nxdata,
   off_t offset = blknum * BLKSIZE;
   enum bm_entry state = bitmap_get_blk (&bm, blknum, BLOCK_NOT_CACHED);
 
+  reclaim ();
+
   nbdkit_debug ("cache: blk_read block %" PRIu64 " (offset
%" PRIu64 ") is %s",
                 blknum, (uint64_t) offset,
                 state == BLOCK_NOT_CACHED ? "not cached" :
@@ -193,6 +222,8 @@ blk_writethrough (struct nbdkit_next_ops *next_ops, void
*nxdata,
 {
   off_t offset = blknum * BLKSIZE;
 
+  reclaim ();
+
   nbdkit_debug ("cache: writethrough block %" PRIu64 " (offset
%" PRIu64 ")",
                 blknum, (uint64_t) offset);
 
@@ -224,6 +255,8 @@ blk_write (struct nbdkit_next_ops *next_ops, void *nxdata,
 
   offset = blknum * BLKSIZE;
 
+  reclaim ();
+
   nbdkit_debug ("cache: writeback block %" PRIu64 " (offset
%" PRIu64 ")",
                 blknum, (uint64_t) offset);
 
@@ -254,3 +287,133 @@ for_each_dirty_block (block_callback f, void *vp)
 
   return 0;
 }
+
+#ifdef HAVE_CACHE_RECLAIM
+
+static void reclaim_one (void);
+static void reclaim_lru (void);
+static void reclaim_any (void);
+static void reclaim_block (void);
+
+/* See if we need to reclaim blocks. */
+static void
+reclaim (void)
+{
+  struct stat statbuf;
+  uint64_t cache_allocated;
+
+  /* If the user didn't set cache-max-size, do nothing. */
+  if (max_size == -1) return;
+
+  /* Check the allocated size of the cache. */
+  if (fstat (fd, &statbuf) == -1) {
+    nbdkit_debug ("cache: fstat: %m");
+    return;
+  }
+  cache_allocated = statbuf.st_blocks * UINT64_C(512);
+
+  if (reclaiming) {
+    /* Keep reclaiming until the cache size drops below the low threshold. */
+    if (cache_allocated < max_size * lo_thresh / 100) {
+      nbdkit_debug ("cache: stop reclaiming");
+      reclaiming = NOT_RECLAIMING;
+      return;
+    }
+  }
+  else {
+    if (cache_allocated < max_size * hi_thresh / 100)
+      return;
+
+    /* Start reclaiming if the cache size goes over the high threshold. */
+    nbdkit_debug ("cache: start reclaiming");
+    reclaiming = RECLAIMING_LRU;
+  }
+
+  /* Reclaim up to 2 cache blocks. */
+  reclaim_one ();
+  reclaim_one ();
+}
+
+/* Reclaim a single cache block. */
+static void
+reclaim_one (void)
+{
+  assert (reclaiming);
+
+  if (reclaiming == RECLAIMING_LRU)
+    reclaim_lru ();
+  else
+    reclaim_any ();
+}
+
+static void
+reclaim_lru (void)
+{
+  uint64_t old_reclaim_blk;
+
+  /* Find the next block in the cache. */
+  reclaim_blk = bitmap_next (&bm, reclaim_blk+1);
+  old_reclaim_blk = reclaim_blk;
+
+  /* Search for an LRU block after this one. */
+  do {
+    if (! lru_has_been_recently_accessed (reclaim_blk)) {
+      reclaim_block ();
+      return;
+    }
+
+    reclaim_blk = bitmap_next (&bm, reclaim_blk+1);
+    if (reclaim_blk == -1)    /* wrap around */
+      reclaim_blk = bitmap_next (&bm, 0);
+  } while (reclaim_blk >= 0 && old_reclaim_blk != reclaim_blk);
+
+  if (old_reclaim_blk == reclaim_blk) {
+    /* Run out of LRU blocks, so start reclaiming any block in the cache. */
+    nbdkit_debug ("cache: reclaiming any blocks");
+    reclaiming = RECLAIMING_ANY;
+    reclaim_one ();
+    return;
+  }
+}
+
+static void
+reclaim_any (void)
+{
+  /* Find the next block in the cache. */
+  reclaim_blk = bitmap_next (&bm, reclaim_blk+1);
+  if (reclaim_blk == -1)        /* wrap around */
+    reclaim_blk = bitmap_next (&bm, 0);
+
+  reclaim_block ();
+}
+
+static void
+reclaim_block (void)
+{
+  if (reclaim_blk == -1) {
+    nbdkit_debug ("cache: run out of blocks to reclaim!");
+    return;
+  }
+
+  nbdkit_debug ("cache: reclaiming block %" PRIu64, reclaim_blk);
+#ifdef FALLOC_FL_PUNCH_HOLE
+  if (fallocate (fd, FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE,
+                 reclaim_blk * BLKSIZE, BLKSIZE) == -1) {
+    nbdkit_error ("cache: reclaiming cache blocks: "
+                  "fallocate: FALLOC_FL_PUNCH_HOLE: %m");
+    return;
+  }
+#else
+#error "no implementation for punching holes"
+#endif
+
+  bitmap_set_blk (&bm, reclaim_blk, BLOCK_NOT_CACHED);
+}
+
+#else /* !HAVE_CACHE_RECLAIM */
+static void
+reclaim (void)
+{
+  /* nothing */
+}
+#endif /* !HAVE_CACHE_RECLAIM */
diff --git a/filters/cache/cache.c b/filters/cache/cache.c
index a14f789..67dde23 100644
--- a/filters/cache/cache.c
+++ b/filters/cache/cache.c
@@ -65,6 +65,8 @@
 static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
 
 enum cache_mode cache_mode = CACHE_MODE_WRITEBACK;
+int64_t max_size = -1;
+int hi_thresh = 95, lo_thresh = 80;
 bool cache_on_read = false;
 
 static int cache_flush (struct nbdkit_next_ops *next_ops, void *nxdata, void
*handle, uint32_t flags, int *err);
@@ -105,6 +107,44 @@ cache_config (nbdkit_next_config *next, void *nxdata,
       return -1;
     }
   }
+#ifdef HAVE_CACHE_RECLAIM
+  else if (strcmp (key, "cache-max-size") == 0) {
+    int64_t r;
+
+    r = nbdkit_parse_size (value);
+    if (r == -1)
+      return -1;
+    /* We set a lower limit for the cache size just to keep out of
+     * trouble.
+     */
+    if (r < 1024*1024) {
+      nbdkit_error ("cache-max-size is too small");
+      return -1;
+    }
+    max_size = r;
+    return 0;
+  }
+  else if (strcmp (key, "cache-high-threshold") == 0) {
+    if (sscanf (value, "%d", &hi_thresh) != 1) {
+      nbdkit_error ("invalid cache-high-threshold parameter: %s",
value);
+      return -1;
+    }
+    if (hi_thresh <= 0) {
+      nbdkit_error ("cache-high-threshold must be greater than
zero");
+    }
+    return 0;
+  }
+  else if (strcmp (key, "cache-low-threshold") == 0) {
+    if (sscanf (value, "%d", &lo_thresh) != 1) {
+      nbdkit_error ("invalid cache-low-threshold parameter: %s",
value);
+      return -1;
+    }
+    if (lo_thresh <= 0) {
+      nbdkit_error ("cache-low-threshold must be greater than zero");
+    }
+    return 0;
+  }
+#endif
   else if (strcmp (key, "cache-on-read") == 0) {
     int r;
 
@@ -119,6 +159,21 @@ cache_config (nbdkit_next_config *next, void *nxdata,
   }
 }
 
+static int
+cache_config_complete (nbdkit_next_config_complete *next, void *nxdata)
+{
+  /* If cache-max-size was set then check the thresholds. */
+  if (max_size != -1) {
+    if (lo_thresh >= hi_thresh) {
+      nbdkit_error ("cache-low-threshold must be "
+                    "less than cache-high-threshold");
+      return -1;
+    }
+  }
+
+  return next (nxdata);
+}
+
 static void *
 cache_open (nbdkit_next_open *next, void *nxdata, int readonly)
 {
@@ -416,6 +471,7 @@ static struct nbdkit_filter filter = {
   .load              = cache_load,
   .unload            = cache_unload,
   .config            = cache_config,
+  .config_complete   = cache_config_complete,
   .open              = cache_open,
   .prepare           = cache_prepare,
   .get_size          = cache_get_size,
diff --git a/filters/cache/lru.c b/filters/cache/lru.c
index 60cf379..8159040 100644
--- a/filters/cache/lru.c
+++ b/filters/cache/lru.c
@@ -109,8 +109,11 @@ lru_set_size (uint64_t new_size)
   if (bitmap_resize (&bm[1], new_size) == -1)
     return -1;
 
-  /* XXX Choose this better. */
-  N = MAX (new_size / BLKSIZE / 4, 100);
+  if (max_size != -1)
+    /* Make the threshold about 1/4 the maximum size of the cache. */
+    N = MAX (max_size / BLKSIZE / 4, 100);
+  else
+    N = MAX (new_size / BLKSIZE / 4, 100);
 
   return 0;
 }
diff --git a/TODO b/TODO
index 9d6e727..7107ea9 100644
--- a/TODO
+++ b/TODO
@@ -100,15 +100,8 @@ Suggestions for filters
 * masking plugin features for testing clients (see 'nozero' and
'fua'
   filters for examples)
 
-nbdkit-cache-filter needs considerable work:
-
-* allow the user to limit the total size of the cache
-
-* handle ENOSPC errors
-
-* implement some cache replacement policies
-
-* some sort of background task or thread to write out dirty blocks
+* nbdkit-cache-filter should handle ENOSPC errors automatically by
+  reclaiming blocks from the cache
 
 Composing nbdkit
 ----------------
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 03cd206..1e6049f 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -42,6 +42,7 @@ EXTRA_DIST = \
 	shebang.rb \
 	test-blocksize.sh \
 	test-cache.sh \
+	test-cache-max-size.sh \
 	test-cache-on-read.sh \
 	test-captive.sh \
 	test-cow.sh \
@@ -709,7 +710,8 @@ TESTS += test-blocksize.sh
 if HAVE_GUESTFISH
 TESTS += \
 	test-cache.sh \
-	test-cache-on-read.sh
+	test-cache-on-read.sh \
+	test-cache-max-size.sh
 endif HAVE_GUESTFISH
 
 # cow filter test.
diff --git a/tests/test-cache-max-size.sh b/tests/test-cache-max-size.sh
new file mode 100755
index 0000000..4d34dbc
--- /dev/null
+++ b/tests/test-cache-max-size.sh
@@ -0,0 +1,96 @@
+#!/usr/bin/env bash
+# nbdkit
+# Copyright (C) 2018 Red Hat Inc.
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#
+# * Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+#
+# * Neither the name of Red Hat nor the names of its contributors may be
+# used to endorse or promote products derived from this software without
+# specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+# USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+
+source ./functions.sh
+set -e
+set -x
+
+# Check that this is a Linux-like system supporting /proc/pid/fd.
+if ! test -d /proc/self/fd; then
+    echo "$0: not a Linux-like system supporting /proc/pid/fd"
+    exit 77
+fi
+
+# Test that qemu-io works
+if ! qemu-io --help >/dev/null; then
+    echo "$0: missing or broken qemu-io"
+    exit 77
+fi
+
+# Need the stat command from coreutils.
+if ! stat --version >/dev/null; then
+    echo "$0: missing or broken stat command"
+    exit 77
+fi
+
+d=cache-max-size.d
+rm -rf $d
+mkdir -p $d
+cleanup_fn rm -rf $d
+
+# Create a cache directory.
+mkdir $d/cache
+TMPDIR=$d/cache
+export TMPDIR
+
+# Create an empty base image.
+truncate -s 1G $d/cache-max-size.img
+
+# Run nbdkit with the caching filter and a low size limit to ensure
+# that the reclaim code is exercised.
+start_nbdkit -P $d/cache-max-size.pid -U $d/cache-max-size.sock \
+             --filter=cache \
+             file $d/cache-max-size.img \
+             cache-max-size=10M cache-on-read=true
+
+# Write > 10M to the plugin.
+qemu-io -f raw "nbd+unix://?socket=$d/cache-max-size.sock" \
+        -c "w -P 0 0 10M" \
+        -c "w -P 0 20M 10M" \
+        -c "r 20M 10M" \
+        -c "r 30M 10M" \
+        -c "w -P 0 40M 10M"
+
+# We can't use ?du? on the cache directory because the cache file is
+# deleted by the filter, and so is only accessible via /proc/PID/fd.
+# Get the /proc link to the cache file, and the size of it in bytes.
+fddir="/proc/$( cat $d/cache-max-size.pid )/fd"
+ls -l $fddir
+fd="$fddir/$( ls -l $fddir | grep $TMPDIR | head -1 | awk '{print
$9}' )"
+stat -L $fd
+size=$(( $(stat -L -c '%b * %B' $fd) ))
+
+if [ "$size" -gt $(( 11 * 1024 * 1024 )) ]; then
+    echo "$0: cache size is larger than 10M (actual size: $size
bytes)"
+    exit 1
+fi
-- 
2.19.2

On 1/1/19 8:33 AM, Richard W.M. Jones wrote:
> It's useful to be able to search for the next non-zero entry in a
> bitmap.  This commit adds a ‘bitmap_next’ function to do that.
> Because the bitmap is just a uint8_t buffer, using fast string
> functions we should be able to do this quickly even if the bitmap is
> sparse.  (However the actual implementation is not optimized since
> that is quite complicated - see to-do comments in
> common/include/nextnonzero.h).
> 
> I wasn't confident about the correctness of the code and so this
> commit also adds some unit tests covering all of the bitmap code.
> ---
> +#define NBDKIT_NEXTNONZERO_H
> +
> +/* Given a byte buffer, return a pointer to the first non-zero byte,
> + * or return NULL if we reach the end of the buffer.
> + *
> + * XXX: Even with -O3 -mavx2, gcc 8.2.1 makes a terrible job with this
maybe s/makes/does/
> + * loop, compiling it completely naively.  QEMU has an AVX2
> + * implementation of a similar loop.
> + *
> +/* Unit tests of the bitmap code. */
> +
> +
> +  /* Check the values of all bits. */
> +  for (i = j = 0; i < nr_blocks; ++i) {
> +    if (i == blks[j]) { /* previously set bit */
> +      vexp = (j & 1) == 0 ? 1 : (1<<bpb) - 1;
> +      v = bitmap_get_blk (&bm, blks[j], 0);
> +      assert (v == vexp);
> +      ++j;
> +    }
> +    else { /* unset bit, except it to be zero */
s/except/expect/
> +      v = bitmap_get_blk (&bm, i, 0);
> +      assert (v == 0);
> +    }
> +  }
> +
Otherwise looks good.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

On 1/1/19 8:33 AM, Richard W.M. Jones wrote:
> ---
>  filters/cache/lru.h       |  54 ++++++++++++++
>  filters/cache/blk.c       |  12 +++
>  filters/cache/lru.c       | 150 ++++++++++++++++++++++++++++++++++++++
>  filters/cache/Makefile.am |   2 +
>  4 files changed, 218 insertions(+)
> 
> +++ b/filters/cache/lru.c
> @@ -0,0 +1,150 @@
> +/* nbdkit
> + * Copyright (C) 2018 Red Hat Inc.
Always fun to see new files added near a new year boundary. Up to you if
you want to add 2019.

> +/* These are the block operations.  They always read or write a single
> + * whole block of size ‘blksize’.
> + */
> +
> +#include <config.h>
> +
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <stdint.h>
> +#include <stdbool.h>
> +#include <inttypes.h>
> +
> +#include <nbdkit-filter.h>
> +
> +#include "bitmap.h"
> +
> +#include "cache.h"
> +#include "blk.h"
> +#include "lru.h"
> +
> +#define MAX(a, b) ((a) > (b) ? (a) : (b))
> +
> +/* LRU bitmaps.  These bitmaps implement a simple, fast LRU structure.
> + *
> + *    bm[0]         bm[1]         blocks not in either bitmap
> + * ┌─────────┬──────────────────┬─────────────────────────────┐
> + * │         │                  │                             │
> + * └─────────┴──────────────────┴─────────────────────────────┘
> + *    ↑          c1 bits set
> + *  c0 bits set
> + *
> + * The LRU structure keeps track of the [approx] last N distinct
> + * blocks which have been most recently accessed.  It can answer in
> + * O(1) time the question: “Is a particular block in or not in the N
> + * distinct blocks most recently accessed?”
> + *
> + * To do this we keep two bitmaps.
> + *
> + * When a block is accessed, we set the corresponding bit in bm[0] and
> + * increment c0 (c0 counts the number of bits set in bm[0]).  If c0 =>
+ * N/2 then we swap the two bitmaps, clear bm[0], and reset c0 to 0.
Are you incrementing c0 on every access, or just on accesses that flip a
bit from 0 to 1?  That is, if I access sector 0 repeatedly and nothing
else (say 100 times), is c0 set to 100 or to just 1?
> + *
> + * To check if a block has been accessed within the previous N
> + * distinct accesses, we simply have to check both bitmaps.  If it is
> + * not in either bitmap, then it's old and a candidate to be
> + * reclaimed.
Depending on how c0 is incremented, this is either the last previous N
accesses (even if they were repeats of an already LRU block), or the
last previously used N blocks (even if there were many more than N
accesses). Both methods have their advantages (the former can be used to
reduce memory usage to only the hot blocks, rather than always carrying
a full N blocks in memory; the latter reduces the times the cache has to
be repopulated).
> + *
> + * You'll note that in fact we only keep track of between N/2 and N
> + * recently accessed blocks.  We could make the estimate more accurate
> + * by having more bitmaps, but as this is only a heuristic we choose
> + * to keep the implementation simple and memory usage low instead.
Seems reasonable enough.
> + */
> +static struct bitmap bm[2];
> +static unsigned c0 = 0, c1 = 0;
Static variables already start life zero-initialized.

> +
> +void
> +lru_set_recently_accessed (uint64_t blknum)
> +{
> +  /* If the block is already set in the first bitmap, don't need to do
> +   * anything.
> +   */
> +  if (bitmap_get_blk (&bm[0], blknum, false))
> +    return;
And this implements the latter (the most recent N blocks accessed, not
the most recent N accesses).
> +
> +  bitmap_set_blk (&bm[0], blknum, true);
> +  c0++;
> +
> +  /* If we've reached N/2 then we need to swap over the bitmaps. */
> +  if (c0 >= N/2) {
> +    struct bitmap tmp;
> +
> +    tmp = bm[0];
> +    bm[0] = bm[1];
> +    bm[1] = tmp;
> +    c1 = c0;
> +
> +    bitmap_clear (&bm[0]);
> +    c0 = 0;
> +  }
> +}
> +
> +bool
> +lru_has_been_recently_accessed (uint64_t blknum)
> +{
> +  return
> +    bitmap_get_blk (&bm[0], blknum, false) ||
> +    bitmap_get_blk (&bm[1], blknum, false);
> +}
Code matches the documented algorithm, and looks sane.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

On 1/1/19 8:33 AM, Richard W.M. Jones wrote:
> The original plan was to have a background thread doing the reclaim.
> However that cannot work given the design of filters, because a
> background thread cannot access the next_ops struct which is only
> available during requests.
> 
> Therefore we spread the work over the request threads.  Each blk_*
> function checks whether there is work to do, and if there is will
> reclaim up to two blocks from the cache (thus ensuring that we always
> make forward progress, since each blk_* function can only add at most
> one block to the cache).
> ---
>  filters/cache/nbdkit-cache-filter.pod |  71 +++++++++++
>  filters/cache/cache.h                 |  13 ++
>  filters/cache/blk.c                   | 163 ++++++++++++++++++++++++++
>  filters/cache/cache.c                 |  56 +++++++++
>  filters/cache/lru.c                   |   7 +-
>  TODO                                  |  11 +-
>  tests/Makefile.am                     |   4 +-
>  tests/test-cache-max-size.sh          |  96 +++++++++++++++
>  8 files changed, 409 insertions(+), 12 deletions(-)
> 
> +=back
> +
> +The way this works is once the size of the cache exceeds
> +S<C<SIZE> ✕ the high threshold>, the filter works to reduce
the size
> +of the cache until it is less than S<C<SIZE> ✕ the low
threshold>.
> +Once the size is below the low threshold, no more reclaim work is done
> +until the size exceeds the high threshold again.
> +
> +The default thresholds are high 95% and low 80%.  The thresholds are
> +expressed as percentages of C<cache-max-size>, and may be greater
than
> +100.
Do the thresholds support fractional percentages, like 90.5, or must
they be integral?  Should we enforce that low threshold < high
threshold?  What does it really mean to have a threshold > 100 - that we
can never reach the high threshold?
> +/* Do we support reclaiming cache blocks? */
> +#ifdef FALLOC_FL_PUNCH_HOLE
> +#define HAVE_CACHE_RECLAIM 1
> +#else
> +#undef HAVE_CACHE_RECLAIM
> +#endif
> +
Should the man page mention that max cache size can only be enforced
with kernel support?  Do we want to go further and probe whether
FALLOC_FL_PUNCH_HOLE works on $TMPDIR?  (Even if the kernel supports
punching holes, there are some filesystems that don't).

> @@ -74,6 +76,31 @@ enum bm_entry {
>    BLOCK_DIRTY = 3,
>  };
>  
> +#ifdef HAVE_CACHE_RECLAIM
> +/* If we are currently reclaiming blocks from the cache.
> + *
> + * The state machine starts in the NOT_RECLAIMING state.  When the
> + * size of the cache exceeds the high threshold, we move to
> + * RECLAIMING_LRU.  Once we have exhausted all LRU blocks, we move to
> + * RECLAIMING_ANY (reclaiming any blocks).
> + *
> + * If at any time the size of the cache goes below the low threshold
> + * we move back to the NOT_RECLAIMING state.
> + *
> + * reclaim_blk is the last block that we will looked at.
s/will //

> +
> +#ifdef HAVE_CACHE_RECLAIM
> +
> +static void reclaim_one (void);
> +static void reclaim_lru (void);
> +static void reclaim_any (void);
> +static void reclaim_block (void);
> +
> +/* See if we need to reclaim blocks. */
> +static void
> +reclaim (void)
> +{
> +  struct stat statbuf;
> +  uint64_t cache_allocated;
> +
> +  /* If the user didn't set cache-max-size, do nothing. */
> +  if (max_size == -1) return;
> +
> +  /* Check the allocated size of the cache. */
> +  if (fstat (fd, &statbuf) == -1) {
> +    nbdkit_debug ("cache: fstat: %m");
> +    return;
> +  }
> +  cache_allocated = statbuf.st_blocks * UINT64_C(512);
Is that calculation always going to be accurate, even when more
disks/file systems move towards 4k minimum access?
> +
> +  if (reclaiming) {
> +    /* Keep reclaiming until the cache size drops below the low threshold.
*/
> +    if (cache_allocated < max_size * lo_thresh / 100) {
> +      nbdkit_debug ("cache: stop reclaiming");
> +      reclaiming = NOT_RECLAIMING;
> +      return;
> +    }
> +  }
> +  else {
> +    if (cache_allocated < max_size * hi_thresh / 100)
> +      return;
> +
> +    /* Start reclaiming if the cache size goes over the high threshold. */
> +    nbdkit_debug ("cache: start reclaiming");
> +    reclaiming = RECLAIMING_LRU;
> +  }
> +
> +  /* Reclaim up to 2 cache blocks. */
> +  reclaim_one ();
> +  reclaim_one ();
> +}
> +
> +/* Reclaim a single cache block. */
> +static void
> +reclaim_one (void)
> +{
> +  assert (reclaiming);
> +
> +  if (reclaiming == RECLAIMING_LRU)
> +    reclaim_lru ();
> +  else
> +    reclaim_any ();
> +}
> +
> +static void
> +reclaim_lru (void)
> +{
> +  uint64_t old_reclaim_blk;
> +
> +  /* Find the next block in the cache. */
> +  reclaim_blk = bitmap_next (&bm, reclaim_blk+1);
> +  old_reclaim_blk = reclaim_blk;
> +
> +  /* Search for an LRU block after this one. */
> +  do {
> +    if (! lru_has_been_recently_accessed (reclaim_blk)) {
> +      reclaim_block ();
> +      return;
> +    }
> +
> +    reclaim_blk = bitmap_next (&bm, reclaim_blk+1);
> +    if (reclaim_blk == -1)    /* wrap around */
> +      reclaim_blk = bitmap_next (&bm, 0);
> +  } while (reclaim_blk >= 0 && old_reclaim_blk != reclaim_blk);
> +
> +  if (old_reclaim_blk == reclaim_blk) {
> +    /* Run out of LRU blocks, so start reclaiming any block in the cache.
*/
> +    nbdkit_debug ("cache: reclaiming any blocks");
> +    reclaiming = RECLAIMING_ANY;
> +    reclaim_one ();
> +    return;
> +  }
> +}
> +
> +static void
> +reclaim_any (void)
> +{
> +  /* Find the next block in the cache. */
> +  reclaim_blk = bitmap_next (&bm, reclaim_blk+1);
> +  if (reclaim_blk == -1)        /* wrap around */
> +    reclaim_blk = bitmap_next (&bm, 0);
> +
> +  reclaim_block ();
> +}
> +
> +static void
> +reclaim_block (void)
> +{
> +  if (reclaim_blk == -1) {
> +    nbdkit_debug ("cache: run out of blocks to reclaim!");
> +    return;
> +  }
> +
> +  nbdkit_debug ("cache: reclaiming block %" PRIu64,
reclaim_blk);
Where does this prioritize clean blocks over dirty blocks?  Or is the
.pod change inaccurate given later changes to implementation?
> +#ifdef FALLOC_FL_PUNCH_HOLE
> +  if (fallocate (fd, FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE,
> +                 reclaim_blk * BLKSIZE, BLKSIZE) == -1) {
> +    nbdkit_error ("cache: reclaiming cache blocks: "
> +                  "fallocate: FALLOC_FL_PUNCH_HOLE: %m");
> +    return;
> +  }
> +#else
> +#error "no implementation for punching holes"
This #error should be unreachable, given the definition of
HAVE_CACHE_RECLAIM based on FALLOC_FL_PUNCH_HOLE.
> +#endif
> +
> +  bitmap_set_blk (&bm, reclaim_blk, BLOCK_NOT_CACHED);
> +}
> +
> +#else /* !HAVE_CACHE_RECLAIM */
> +static void
> +reclaim (void)
> +{
> +  /* nothing */
> +}
> +#endif /* !HAVE_CACHE_RECLAIM */
> diff --git a/filters/cache/cache.c b/filters/cache/cache.c
> index a14f789..67dde23 100644
> --- a/filters/cache/cache.c
> +++ b/filters/cache/cache.c
> @@ -65,6 +65,8 @@
>  static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
>  
>  enum cache_mode cache_mode = CACHE_MODE_WRITEBACK;
> +int64_t max_size = -1;
> +int hi_thresh = 95, lo_thresh = 80;
>  bool cache_on_read = false;
>  
>  static int cache_flush (struct nbdkit_next_ops *next_ops, void *nxdata,
void *handle, uint32_t flags, int *err);
> @@ -105,6 +107,44 @@ cache_config (nbdkit_next_config *next, void *nxdata,
>        return -1;
>      }
>    }
> +#ifdef HAVE_CACHE_RECLAIM
> +  else if (strcmp (key, "cache-max-size") == 0) {
> +    int64_t r;
> +
> +    r = nbdkit_parse_size (value);
> +    if (r == -1)
> +      return -1;
> +    /* We set a lower limit for the cache size just to keep out of
> +     * trouble.
> +     */
> +    if (r < 1024*1024) {
> +      nbdkit_error ("cache-max-size is too small");
> +      return -1;
> +    }
> +    max_size = r;
> +    return 0;
> +  }
> +  else if (strcmp (key, "cache-high-threshold") == 0) {
> +    if (sscanf (value, "%d", &hi_thresh) != 1) {
> +      nbdkit_error ("invalid cache-high-threshold parameter:
%s", value);
> +      return -1;
> +    }
> +    if (hi_thresh <= 0) {
> +      nbdkit_error ("cache-high-threshold must be greater than
zero");
> +    }
> +    return 0;
> +  }
> +  else if (strcmp (key, "cache-low-threshold") == 0) {
> +    if (sscanf (value, "%d", &lo_thresh) != 1) {
> +      nbdkit_error ("invalid cache-low-threshold parameter: %s",
value);
> +      return -1;
> +    }
> +    if (lo_thresh <= 0) {
> +      nbdkit_error ("cache-low-threshold must be greater than
zero");
> +    }
> +    return 0;
> +  }
> +#endif
There is no #else, so on platforms that can't punch holes, this silently
passes cache-max-size and friends on to the next filter layer, where it
will probably result in some odd error about being unrecognized. Would
it be better to have an explicit #else branch here that specifically
handles (and rejects) these parameters where we can't implement them?
> +++ b/tests/test-cache-max-size.sh
> +
> +# Check that this is a Linux-like system supporting /proc/pid/fd.
> +if ! test -d /proc/self/fd; then
> +    echo "$0: not a Linux-like system supporting /proc/pid/fd"
Would this read any better if you used /proc/\$pid/fd or /proc/PID/fd in
the error message?  Especially since...
> +    exit 77
> +fi
> +
> +# Test that qemu-io works
> +if ! qemu-io --help >/dev/null; then
> +    echo "$0: missing or broken qemu-io"
> +    exit 77
> +fi
> +
> +# Need the stat command from coreutils.
> +if ! stat --version >/dev/null; then
> +    echo "$0: missing or broken stat command"
> +    exit 77
> +fi
> +
> +d=cache-max-size.d
> +rm -rf $d
> +mkdir -p $d
> +cleanup_fn rm -rf $d
> +
> +# Create a cache directory.
> +mkdir $d/cache
> +TMPDIR=$d/cache
> +export TMPDIR
> +
> +# Create an empty base image.
> +truncate -s 1G $d/cache-max-size.img
> +
> +# Run nbdkit with the caching filter and a low size limit to ensure
> +# that the reclaim code is exercised.
> +start_nbdkit -P $d/cache-max-size.pid -U $d/cache-max-size.sock \
> +             --filter=cache \
> +             file $d/cache-max-size.img \
> +             cache-max-size=10M cache-on-read=true
> +
> +# Write > 10M to the plugin.
> +qemu-io -f raw "nbd+unix://?socket=$d/cache-max-size.sock" \
> +        -c "w -P 0 0 10M" \
> +        -c "w -P 0 20M 10M" \
> +        -c "r 20M 10M" \
> +        -c "r 30M 10M" \
> +        -c "w -P 0 40M 10M"
> +
> +# We can't use ‘du’ on the cache directory because the cache file is
> +# deleted by the filter, and so is only accessible via /proc/PID/fd.
...you use PID here to make it obvious it is a placeholder?
> +# Get the /proc link to the cache file, and the size of it in bytes.
> +fddir="/proc/$( cat $d/cache-max-size.pid )/fd"
> +ls -l $fddir
> +fd="$fddir/$( ls -l $fddir | grep $TMPDIR | head -1 | awk '{print
$9}' )"
> +stat -L $fd
> +size=$(( $(stat -L -c '%b * %B' $fd) ))
Wow - that's quite a bit of work. But it looks reasonable and sticks to
tools likely to already be present for other tests, and avoids dragging
in lsof or other less-common dependencies.
> +
> +if [ "$size" -gt $(( 11 * 1024 * 1024 )) ]; then
> +    echo "$0: cache size is larger than 10M (actual size: $size
bytes)"
> +    exit 1
> +fi
> 
-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org