block: limited request size in write zeroes unsupported path

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 <[email protected]>
Signed-off-by: Peter Lieven <[email protected]>
Reviewed-by: Denis V. Lunev <[email protected]>
Message-id: 1420457389[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>

diff --git a/block.c b/block.c
index e76a223eae0b3e2f550e19261790dfe9aaee41de..371d0f6745a7b8646249debcb282ed978f6569c0 100644 (file)
--- a/block.c
+++ b/block.c
@@ -3244,6 +3244,9 @@ 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);
@@ -3260,7 +3263,7 @@ 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;
             }