[Qemu-devel] [PATCH] block: limited request size in write zeroes unsupported path

[Peter Lieven <pl@kamp.de>]

If bs->bl.max_write_zeroes is large and we end up in the unsupported
path we might allocate a lot of memory for the iovector and/or even
generate an oversized requests.

Fix this by limiting the request by the minimum of the reported
maximum transfer size or 16MB (32768 sectors).

Reported-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Peter Lieven <pl@kamp.de>
---
 block.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index a612594..8009478 100644
--- a/block.c
+++ b/block.c
@@ -3203,6 +3203,9 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
 
         if (ret == -ENOTSUP) {
             /* Fall back to bounce buffer if write zeroes is unsupported */
+            int max_xfer_len = MIN_NON_ZERO(bs->bl.max_transfer_length,
+                                            MAX_WRITE_ZEROES_DEFAULT);
+            num = MIN(num, max_xfer_len);
             iov.iov_len = num * BDRV_SECTOR_SIZE;
             if (iov.iov_base == NULL) {
                 iov.iov_base = qemu_try_blockalign(bs, num * BDRV_SECTOR_SIZE);
@@ -3219,7 +3222,7 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
             /* Keep bounce buffer around if it is big enough for all
              * all future requests.
              */
-            if (num < max_write_zeroes) {
+            if (num < max_xfer_len) {
                 qemu_vfree(iov.iov_base);
                 iov.iov_base = NULL;
             }
-- 
1.7.9.5