[Qemu-devel] [PATCH v3 3/9] block: Let write zeroes fallback work even with small max_transfer

[Eric Blake <eblake@redhat.com>]

Commit 443668ca rewrote the write_zeroes logic to guarantee that
an unaligned request never crosses a cluster boundary.  But
in the rewrite, the new code assumed that at most one iteration
would be needed to get to an alignment boundary.

However, it is easy to trigger an assertion failure: the Linux
kernel limits loopback devices to advertise a max_transfer of
only 64k.  Any operation that requires falling back to writes
rather than more efficient zeroing must obey max_transfer during
that fallback, which means an unaligned head may require multiple
iterations of the write fallbacks before reaching the aligned
boundaries, when layering a format with clusters larger than 64k
atop the protocol of file access to a loopback device.

Test case:

$ qemu-img create -f qcow2 -o cluster_size=1M file 10M
$ losetup /dev/loop2 /path/to/file
$ qemu-io -f qcow2 /dev/loop2
qemu-io> w 7m 1k
qemu-io> w -z 8003584 2093056

In fairness to Denis (as the original listed author of the culprit
commit), the faulty logic for at most one iteration is probably all
my fault in reworking his idea.  But the solution is to restore what
was in place prior to that commit: when dealing with an unaligned
head or tail, iterate as many times as necessary while fragmenting
the operation at max_transfer boundaries.

Reported-by: Ed Swierk <eswierk@skyportsystems.com>
CC: qemu-stable@nongnu.org
CC: Denis V. Lunev <den@openvz.org>
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/io.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/block/io.c b/block/io.c
index aa532a5..085ac34 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1214,6 +1214,8 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
     int max_write_zeroes = MIN_NON_ZERO(bs->bl.max_pwrite_zeroes, INT_MAX);
     int alignment = MAX(bs->bl.pwrite_zeroes_alignment,
                         bs->bl.request_alignment);
+    int max_transfer = MIN_NON_ZERO(bs->bl.max_transfer,
+                                    MAX_WRITE_ZEROES_BOUNCE_BUFFER);

     assert(alignment % bs->bl.request_alignment == 0);
     head = offset % alignment;
@@ -1229,9 +1231,12 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
          * boundaries.
          */
         if (head) {
-            /* Make a small request up to the first aligned sector.  */
-            num = MIN(count, alignment - head);
-            head = 0;
+            /* Make a small request up to the first aligned sector. For
+             * convenience, limit this request to max_transfer even if
+             * we don't need to fall back to writes.  */
+            num = MIN(MIN(count, max_transfer), alignment - head);
+            head = (head + num) % alignment;
+            assert(num < max_write_zeroes);
         } else if (tail && num > alignment) {
             /* Shorten the request to the last aligned sector.  */
             num -= tail;
@@ -1257,8 +1262,6 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,

         if (ret == -ENOTSUP) {
             /* Fall back to bounce buffer if write zeroes is unsupported */
-            int max_transfer = MIN_NON_ZERO(bs->bl.max_transfer,
-                                            MAX_WRITE_ZEROES_BOUNCE_BUFFER);
             BdrvRequestFlags write_flags = flags & ~BDRV_REQ_ZERO_WRITE;

             if ((flags & BDRV_REQ_FUA) &&
-- 
2.7.4