Libguestfs - Jan 2019 - [PATCH nbdkit v4 2/2] cache: Implement cache-max-size and method of reclaiming space from the cache.

v3 was broken by a bad rebase, so let's forget about that one.

Compared to v2:

 - Patch 1 is the same except for a minor comment change.

 - Patch 2 splits the reclaim code into a separate file
   (filters/cache/reclaim.c)

 - Addressed Eric's comments from his review of v2.

 - Retested on Linux and FreeBSD.

---
 filters/cache/lru.h       |  54 ++++++++++++++
 filters/cache/blk.c       |  12 +++
 filters/cache/lru.c       | 151 ++++++++++++++++++++++++++++++++++++++
 filters/cache/Makefile.am |   2 +
 4 files changed, 219 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..c828968
--- /dev/null
+++ b/filters/cache/lru.c
@@ -0,0 +1,151 @@
+/* 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 new 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 |  50 ++++++
 filters/cache/cache.h                 |   6 +
 filters/cache/reclaim.h               |  48 ++++++
 filters/cache/blk.c                   |   9 +-
 filters/cache/cache.c                 |  66 ++++++++
 filters/cache/lru.c                   |   7 +-
 filters/cache/reclaim.c               | 209 ++++++++++++++++++++++++++
 TODO                                  |  11 +-
 filters/cache/Makefile.am             |   2 +
 tests/Makefile.am                     |   4 +-
 tests/test-cache-max-size.sh          |  96 ++++++++++++
 11 files changed, 495 insertions(+), 13 deletions(-)

diff --git a/filters/cache/nbdkit-cache-filter.pod
b/filters/cache/nbdkit-cache-filter.pod
index 09dc39f..ae2fb42 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 +80,45 @@ 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.
+
+This requires kernel and filesystem support (for L<fallocate(2)>
+C<FALLOC_FL_PUNCH_HOLE>), so it may not work on all platforms.
+
+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%.  You must set
+S<0 E<lt> low E<lt> high>.  The thresholds are expressed as
integer
+percentages of C<cache-max-size>.
+
+Least recently used blocks are discarded first.
+
 =head1 ENVIRONMENT VARIABLES
 
 =over 4
diff --git a/filters/cache/cache.h b/filters/cache/cache.h
index 50416b1..5e0d11b 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
@@ -48,6 +50,10 @@ 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;
 
diff --git a/filters/cache/reclaim.h b/filters/cache/reclaim.h
new file mode 100644
index 0000000..a11a9f3
--- /dev/null
+++ b/filters/cache/reclaim.h
@@ -0,0 +1,48 @@
+/* nbdkit
+ * Copyright (C) 2018-2019 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_RECLAIM_H
+#define NBDKIT_RECLAIM_H
+
+#include "bitmap.h"
+
+/* Do we support reclaiming cache blocks? */
+#ifdef FALLOC_FL_PUNCH_HOLE
+#define HAVE_CACHE_RECLAIM 1
+#else
+#undef HAVE_CACHE_RECLAIM
+#endif
+
+extern void reclaim (int fd, struct bitmap *bm);
+
+#endif /* NBDKIT_RECLAIM_H */
diff --git a/filters/cache/blk.c b/filters/cache/blk.c
index b256446..028eedb 100644
--- a/filters/cache/blk.c
+++ b/filters/cache/blk.c
@@ -54,6 +54,7 @@
 #include "cache.h"
 #include "blk.h"
 #include "lru.h"
+#include "reclaim.h"
 
 /* The cache. */
 static int fd = -1;
@@ -69,7 +70,7 @@ static int fd = -1;
 static struct bitmap bm;
 
 enum bm_entry {
-  BLOCK_NOT_CACHED = 0,
+  BLOCK_NOT_CACHED = 0, /* assumed to be zero by reclaim code */
   BLOCK_CLEAN = 1,
   BLOCK_DIRTY = 3,
 };
@@ -146,6 +147,8 @@ 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 (fd, &bm);
+
   nbdkit_debug ("cache: blk_read block %" PRIu64 " (offset
%" PRIu64 ") is %s",
                 blknum, (uint64_t) offset,
                 state == BLOCK_NOT_CACHED ? "not cached" :
@@ -193,6 +196,8 @@ blk_writethrough (struct nbdkit_next_ops *next_ops, void
*nxdata,
 {
   off_t offset = blknum * BLKSIZE;
 
+  reclaim (fd, &bm);
+
   nbdkit_debug ("cache: writethrough block %" PRIu64 " (offset
%" PRIu64 ")",
                 blknum, (uint64_t) offset);
 
@@ -224,6 +229,8 @@ blk_write (struct nbdkit_next_ops *next_ops, void *nxdata,
 
   offset = blknum * BLKSIZE;
 
+  reclaim (fd, &bm);
+
   nbdkit_debug ("cache: writeback block %" PRIu64 " (offset
%" PRIu64 ")",
                 blknum, (uint64_t) offset);
 
diff --git a/filters/cache/cache.c b/filters/cache/cache.c
index a14f789..569b377 100644
--- a/filters/cache/cache.c
+++ b/filters/cache/cache.c
@@ -56,6 +56,7 @@
 
 #include "cache.h"
 #include "blk.h"
+#include "reclaim.h"
 
 #define THREAD_MODEL NBDKIT_THREAD_MODEL_PARALLEL
 
@@ -65,6 +66,8 @@
 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 +108,53 @@ 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 -1;
+    }
+    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 -1;
+    }
+    return 0;
+  }
+#else /* !HAVE_CACHE_RECLAIM */
+  else if (strcmp (key, "cache-max-size") == 0 ||
+           strcmp (key, "cache-high-threshold") == 0 ||
+           strcmp (key, "cache-low-threshold") == 0) {
+    nbdkit_error ("this platform does not support cache reclaim");
+    return -1;
+  }
+#endif /* !HAVE_CACHE_RECLAIM */
   else if (strcmp (key, "cache-on-read") == 0) {
     int r;
 
@@ -119,6 +169,21 @@ 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 +481,7 @@ 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 c828968..2f379d1 100644
--- a/filters/cache/lru.c
+++ b/filters/cache/lru.c
@@ -110,8 +110,11 @@ 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/filters/cache/reclaim.c b/filters/cache/reclaim.c
new file mode 100644
index 0000000..e57e3ad
--- /dev/null
+++ b/filters/cache/reclaim.c
@@ -0,0 +1,209 @@
+/* nbdkit
+ * Copyright (C) 2018-2019 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.
+ */
+
+#include <config.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include <inttypes.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+
+#include <nbdkit-filter.h>
+
+#include "bitmap.h"
+
+#include "cache.h"
+#include "reclaim.h"
+#include "lru.h"
+
+#ifndef HAVE_CACHE_RECLAIM
+
+void
+reclaim (int fd, struct bitmap *bm)
+{
+  /* nothing */
+}
+
+#else /* 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.
+ *
+ * A possible future enhancement is to add an extra state between LRU
+ * and ANY which reclaims blocks from lru.c:bm[1].
+ *
+ * reclaim_blk is the last block that we 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;
+
+static void reclaim_one (int fd, struct bitmap *bm);
+static void reclaim_lru (int fd, struct bitmap *bm);
+static void reclaim_any (int fd, struct bitmap *bm);
+static void reclaim_block (int fd, struct bitmap *bm);
+
+/* See if we need to reclaim blocks. */
+void
+reclaim (int fd, struct bitmap *bm)
+{
+  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 (fd, bm);
+  reclaim_one (fd, bm);
+}
+
+/* Reclaim a single cache block. */
+static void
+reclaim_one (int fd, struct bitmap *bm)
+{
+  assert (reclaiming);
+
+  if (reclaiming == RECLAIMING_LRU)
+    reclaim_lru (fd, bm);
+  else
+    reclaim_any (fd, bm);
+}
+
+static void
+reclaim_lru (int fd, struct bitmap *bm)
+{
+  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 (fd, bm);
+      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 (fd, bm);
+    return;
+  }
+}
+
+static void
+reclaim_any (int fd, struct bitmap *bm)
+{
+  /* 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 (fd, bm);
+}
+
+static void
+reclaim_block (int fd, struct bitmap *bm)
+{
+  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, 0);
+}
+
+#endif /* HAVE_CACHE_RECLAIM */
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/filters/cache/Makefile.am b/filters/cache/Makefile.am
index 1620fa1..1d167aa 100644
--- a/filters/cache/Makefile.am
+++ b/filters/cache/Makefile.am
@@ -43,6 +43,8 @@ nbdkit_cache_filter_la_SOURCES = \
 	cache.h \
 	lru.c \
 	lru.h \
+	reclaim.c \
+	reclaim.h \
 	$(top_srcdir)/include/nbdkit-filter.h
 
 nbdkit_cache_filter_la_CPPFLAGS = \
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..9f2fc88
--- /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 1/3/19 6:37 AM, Richard W.M. Jones wrote:
> ---
No commit body? I like to always have a one-line 'what' in the subject,
and a 'why', however brief, in the commit body.  But for new code
addition, the why can be implied ("for use in the next patch").
>  filters/cache/lru.h       |  54 ++++++++++++++
>  filters/cache/blk.c       |  12 +++
>  filters/cache/lru.c       | 151 ++++++++++++++++++++++++++++++++++++++
>  filters/cache/Makefile.am |   2 +
>  4 files changed, 219 insertions(+)
> 
Looks good now.

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

On 1/3/19 6:37 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).
> ---
> +=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.
Might change in the future if we add support for automatic ENOSPC
handling - but no need to document that until we make such a change.
> +
> +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.
> +
> +This requires kernel and filesystem support (for L<fallocate(2)>
> +C<FALLOC_FL_PUNCH_HOLE>), so it may not work on all platforms.
> +++ 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
Question - how on earth is this supposed to work when we run the cache
with BLKSIZE 4096 - punching holes typically is a no-op unless you have
a large enough block to actually warrant a hole in the file, and I don't
know that 4k is big enough.  Does the kernel coalesce fallocate() calls
over consecutive regions of a file until it has enough to actually punch
a hole?  In which case, your test might happen to show that it "works"
in that you end up punching some holes due to large blocks of
consecutive less-frequently-used clusters after several combined
fallocate() calls; but a different test that is truly random-access will
be dirtying enough sub-hole regions that you can't reclaim anything?  Or
do you need the cache module to use a larger block size by default, or
even add a parameter to control the block size?

> +
> +/* 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.
> + *
> + * A possible future enhancement is to add an extra state between LRU
> + * and ANY which reclaims blocks from lru.c:bm[1].
I'm trying to envision how much thrashing we might encounter (such as
reclaiming a block that the very next blk_*() call pulls back into the
cache), particularly if we don't try to sort out bm[1] first. But while
I can spot obvious one-shot inefficiencies, I haven't been able to
describe a case that will consistently lead to degraded performance due
to churn (since we cycle through the entire image before the next
attempt to reclaim a portion of the image that was just reclaimed).  So
I'm okay with this patch for now (we can always improve things later if
profiling says we need to).

> +/* Reclaim a single cache block. */
> +static void
> +reclaim_one (int fd, struct bitmap *bm)
> +{
> +  assert (reclaiming);
> +
> +  if (reclaiming == RECLAIMING_LRU)
> +    reclaim_lru (fd, bm);
> +  else
> +    reclaim_any (fd, bm);
> +}
> +
> +static void
> +reclaim_lru (int fd, struct bitmap *bm)
> +{
> +  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 (fd, bm);
> +      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 (fd, bm);
Could also call reclaim_any() rather than reclaim_one, since that's all
the more that reclaim_one() will do.
> +    return;
> +  }
> +}
That return doesn't hurt from the maintenance point of view (if you add
later code in the function), but it also isn't doing anything.
> +
> +static void
> +reclaim_any (int fd, struct bitmap *bm)
> +{
> +  /* 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);
Is this code thread-safe?  Now that you are using
NBDKIT_THREAD_MODEL_PARALLEL for the cache filter, shouldn't all
read/write access to the global reclaim_blk be locked, to avoid two
threads trying to act on the same block at once?  I guess it is, because
you only call these functions from the blk_*() functions, and those in
turn are only called from cache.c with the lock held.  But is it worth
documenting the locking constraint somewhere as a comment?
> +
> +  reclaim_block (fd, bm);
> +}
> +
> +static void
> +reclaim_block (int fd, struct bitmap *bm)
> +{
> +  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");
fallocate() fails with EINVAL if BLKSIZE is not a multiple of the
filesystem block size, which gets back to my question of how this can
even work on something that mandates 4k or even 64k access, if the cache
block size is not configurable.

> +source ./functions.sh
> +set -e
> +set -x
> +
> +# Check that this is a Linux-like system supporting /proc/pid/fd.
s/pid/PID/
> +if ! test -d /proc/self/fd; then
> +    echo "$0: not a Linux-like system supporting /proc/\$pid/fd"
> +    exit 77
> +fi
> +
Looking better, but I still have enough questions that may require more
work before you commit.

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