The XFS kernel driver has a bug that may cause data corruption for qcow2
images as of qemu commit c8bb23cbdbe32f. We can work around it by
treating post-EOF fallocates as serializing up until infinity (INT64_MAX
in practice).
Signed-off-by: Max Reitz <mre...@redhat.com>
---
block/file-posix.c | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)
diff --git a/block/file-posix.c b/block/file-posix.c
index 5cd54c8bff..1f5a01df70 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2713,6 +2713,48 @@ raw_do_pwrite_zeroes(BlockDriverState *bs, int64_t
offset, int bytes,
RawPosixAIOData acb;
ThreadPoolFunc *handler;
+#ifdef CONFIG_FALLOCATE
+ if (s->is_xfs && s->use_linux_aio &&
+ offset + bytes > bs->total_sectors * BDRV_SECTOR_SIZE)
+ {
+ BdrvTrackedRequest *req;
+ uint64_t end;
+
+ /*
+ * The Linux XFS driver has a bug where it will discard writes
+ * submitted through the AIO interface if they happen beyond a
+ * concurrently running fallocate() that increases the file
+ * length (i.e., both the write and the fallocate() happen
+ * beyond the EOF).
+ *
+ * To work around it, we look for the tracked request for this
+ * zero write, extend it until INT64_MAX (effectively
+ * infinity), and mark it as serializing.
+ *
+ * TODO: Detect whether this has been fixed in the XFS driver.
+ */
+
+ QLIST_FOREACH(req, &bs->tracked_requests, list) {
+ if (req->co == qemu_coroutine_self() &&
+ req->type == BDRV_TRACKED_WRITE)
+ {
+ break;
+ }
+ }
+
+ assert(req);
+ assert(req->offset <= offset);
+ assert(req->offset + req->bytes >= offset + bytes);
+
+ end = INT64_MAX & -(uint64_t)bs->bl.request_alignment;
+ req->bytes = end - req->offset;
+ req->overlap_bytes = req->bytes;
+
+ bdrv_mark_request_serialising(req, bs->bl.request_alignment);
+ bdrv_wait_serialising_requests(req);
+ }
+#endif
+
acb = (RawPosixAIOData) {
.bs = bs,
.aio_fildes = s->fd,
--
2.21.0
