Eric Blake
2019-May-09 16:02 UTC
[Libguestfs] [nbdkit PATCH] plugins: Use static buffer for plugin_zeroes
No need to calloc/free a buffer every time NBD_CMD_WRITE_ZEROES has to fall back to a .pread call. Just reserve the maximum buffer up front in our bss. Signed-off-by: Eric Blake <eblake@redhat.com> --- I noticed that buf was a candidate for CLEANUP_FREE, but then further noticed that we can avoid the calloc/free altogether if we don't mind the bss being 64M larger. server/plugins.c | 10 ++-------- 1 file changed, 2 insertions(+), 8 deletions(-) diff --git a/server/plugins.c b/server/plugins.c index 947bb6d..acdfa1f 100644 --- a/server/plugins.c +++ b/server/plugins.c @@ -594,7 +594,7 @@ plugin_zero (struct backend *b, struct connection *conn, uint32_t count, uint64_t offset, uint32_t flags, int *err) { struct backend_plugin *p = container_of (b, struct backend_plugin, backend); - char *buf; + static const char buf[MAX_REQUEST_SIZE]; uint32_t limit; int r = -1; bool may_trim = flags & NBDKIT_FLAG_MAY_TRIM; @@ -639,12 +639,7 @@ plugin_zero (struct backend *b, struct connection *conn, assert (p->plugin.pwrite || p->plugin._pwrite_old); flags &= ~NBDKIT_FLAG_MAY_TRIM; threadlocal_set_error (0); - limit = count < MAX_REQUEST_SIZE ? count : MAX_REQUEST_SIZE; - buf = calloc (limit, 1); - if (!buf) { - *err = ENOMEM; - return -1; - } + limit = count < sizeof (buf) ? count : sizeof (buf); while (count) { r = plugin_pwrite (b, conn, buf, limit, offset, flags, err); @@ -656,7 +651,6 @@ plugin_zero (struct backend *b, struct connection *conn, } *err = errno; - free (buf); errno = *err; done: -- 2.20.1
Richard W.M. Jones
2019-May-13 07:43 UTC
Re: [Libguestfs] [nbdkit PATCH] plugins: Use static buffer for plugin_zeroes
On Thu, May 09, 2019 at 11:02:54AM -0500, Eric Blake wrote:> No need to calloc/free a buffer every time NBD_CMD_WRITE_ZEROES has to > fall back to a .pread call. Just reserve the maximum buffer up front > in our bss. > > Signed-off-by: Eric Blake <eblake@redhat.com> > --- > > I noticed that buf was a candidate for CLEANUP_FREE, but then further > noticed that we can avoid the calloc/free altogether if we don't mind > the bss being 64M larger. > > server/plugins.c | 10 ++-------- > 1 file changed, 2 insertions(+), 8 deletions(-) > > diff --git a/server/plugins.c b/server/plugins.c > index 947bb6d..acdfa1f 100644 > --- a/server/plugins.c > +++ b/server/plugins.c > @@ -594,7 +594,7 @@ plugin_zero (struct backend *b, struct connection *conn, > uint32_t count, uint64_t offset, uint32_t flags, int *err) > { > struct backend_plugin *p = container_of (b, struct backend_plugin, backend); > - char *buf; > + static const char buf[MAX_REQUEST_SIZE]; > uint32_t limit; > int r = -1; > bool may_trim = flags & NBDKIT_FLAG_MAY_TRIM; > @@ -639,12 +639,7 @@ plugin_zero (struct backend *b, struct connection *conn, > assert (p->plugin.pwrite || p->plugin._pwrite_old); > flags &= ~NBDKIT_FLAG_MAY_TRIM; > threadlocal_set_error (0); > - limit = count < MAX_REQUEST_SIZE ? count : MAX_REQUEST_SIZE; > - buf = calloc (limit, 1); > - if (!buf) { > - *err = ENOMEM; > - return -1; > - } > + limit = count < sizeof (buf) ? count : sizeof (buf); > > while (count) { > r = plugin_pwrite (b, conn, buf, limit, offset, flags, err); > @@ -656,7 +651,6 @@ plugin_zero (struct backend *b, struct connection *conn, > } > > *err = errno; > - free (buf); > errno = *err; > > done: > --This is much simpler than it sounded when you described it to me on IRC, ACK :-) Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/
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