block: fix preallocate filter: don't do unaligned preallocate requests

There is a bug in handling BDRV_REQ_NO_WAIT flag: we still may wait in
wait_serialising_requests() if request is unaligned. And this is
possible for the only user of this flag (preallocate filter) if
underlying file is unaligned to its request_alignment on start.

So, we have to fix preallocate filter to do only aligned preallocate
requests.

Next, we should fix generic block/io.c somehow. Keeping in mind that
preallocate is the only user of BDRV_REQ_NO_WAIT and that we have to
fix its behavior now, it seems more safe to just assert that we never
use BDRV_REQ_NO_WAIT with unaligned requests and add corresponding
comment. Let's do so.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Message-Id: <20220215121609.38570-1-vsementsov@virtuozzo.com>
[hreitz: Rebased on block GS/IO split]
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

1 files changed, 12 insertions, 3 deletions

diff --git a/block/preallocate.c b/block/preallocate.c
index 1d4233f730..e15cb8c74a 100644
--- a/block/preallocate.c
+++ b/block/preallocate.c
@@ -276,6 +276,10 @@ static bool coroutine_fn handle_write(BlockDriverState *bs, int64_t offset,
     int64_t end = offset + bytes;
     int64_t prealloc_start, prealloc_end;
     int ret;
+    uint32_t file_align = bs->file->bs->bl.request_alignment;
+    uint32_t prealloc_align = MAX(s->opts.prealloc_align, file_align);
+
+    assert(QEMU_IS_ALIGNED(prealloc_align, file_align));
 
     if (!has_prealloc_perms(bs)) {
         /* We don't have state neither should try to recover it */
@@ -320,9 +324,14 @@ static bool coroutine_fn handle_write(BlockDriverState *bs, int64_t offset,
 
     /* Now we want new preallocation, as request writes beyond s->file_end. */
 
-    prealloc_start = want_merge_zero ? MIN(offset, s->file_end) : s->file_end;
-    prealloc_end = QEMU_ALIGN_UP(end + s->opts.prealloc_size,
-                                 s->opts.prealloc_align);
+    prealloc_start = QEMU_ALIGN_UP(
+            want_merge_zero ? MIN(offset, s->file_end) : s->file_end,
+            file_align);
+    prealloc_end = QEMU_ALIGN_UP(
+            MAX(prealloc_start, end) + s->opts.prealloc_size,
+            prealloc_align);
+
+    want_merge_zero = want_merge_zero && (prealloc_start <= offset);
 
     ret = bdrv_co_pwrite_zeroes(
             bs->file, prealloc_start, prealloc_end - prealloc_start,