[Qemu-devel] [PATCH 1/7] block: fix maximum length sent to bdrv_co_do_write_zeroes callback in bs

["Denis V. Lunev" <den@openvz.org>]

The check for maximum length was added by
  commit c31cb70728d2c0c8900b35a66784baa446fd5147
  Author: Peter Lieven <pl@kamp.de>
  Date:   Thu Oct 24 12:06:58 2013 +0200
    block: honour BlockLimits in bdrv_co_do_write_zeroes

but actually if driver provides .bdrv_co_write_zeroes callback, there is
no need to limit the size to 32 MB. Callback should provide effective
implementation which normally should not write any zeroes in comparable
amount.

Correct check should be as follows to honour BlockLimits for slow writes:
- if bs->bl.max_write_zeroes is specified, it should be applied to all
  paths (fast and slow)
- MAX_WRITE_ZEROES_DEFAULT should be applied for slow path only

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Peter Lieven <pl@kamp.de>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
---
 block.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/block.c b/block.c
index 4165d42..0ec8b15 100644
--- a/block.c
+++ b/block.c
@@ -3182,8 +3182,12 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
     struct iovec iov = {0};
     int ret = 0;
 
-    int max_write_zeroes = bs->bl.max_write_zeroes ?
-                           bs->bl.max_write_zeroes : MAX_WRITE_ZEROES_DEFAULT;
+    int max_fast_write_size = nb_sectors;
+    int max_slow_write_size = MAX_WRITE_ZEROES_DEFAULT;
+    if (bs->bl.max_write_zeroes != 0) {
+        max_fast_write_size = bs->bl.max_write_zeroes;
+        max_slow_write_size = bs->bl.max_write_zeroes;
+    }
 
     while (nb_sectors > 0 && !ret) {
         int num = nb_sectors;
@@ -3205,9 +3209,9 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
             }
         }
 
-        /* limit request size */
-        if (num > max_write_zeroes) {
-            num = max_write_zeroes;
+        /* limit request size for fast path */
+        if (num > max_fast_write_size) {
+            num = max_fast_write_size;
         }
 
         ret = -ENOTSUP;
@@ -3217,6 +3221,11 @@ static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
         }
 
         if (ret == -ENOTSUP) {
+            /* limit request size further for slow path */
+            if (num > max_slow_write_size) {
+                num = max_slow_write_size;
+            }
+
             /* Fall back to bounce buffer if write zeroes is unsupported */
             iov.iov_len = num * BDRV_SECTOR_SIZE;
             if (iov.iov_base == NULL) {
@@ -3234,7 +3243,7 @@ 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_slow_write_size) {
                 qemu_vfree(iov.iov_base);
                 iov.iov_base = NULL;
             }
-- 
1.9.1