The XFS kernel driver has a bug that may cause data loss when using
fallocate() in an I/O path, i.e. writing zeroes. 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 | 38 ++++++++++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)
diff --git a/block/file-posix.c b/block/file-posix.c
index 0b7e904d48..d5460f3e45 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2721,6 +2721,44 @@ raw_do_pwrite_zeroes(BlockDriverState *bs, int64_t
offset, int bytes,
RawPosixAIOData acb;
ThreadPoolFunc *handler;
+#ifdef CONFIG_FALLOCATE
+ if (offset + bytes > bs->total_sectors * BDRV_SECTOR_SIZE) {
+ BdrvTrackedRequest *req;
+ uint64_t end;
+
+ /*
+ * This is a workaround for a bug in the Linux XFS driver,
+ * where writes submitted through the AIO interface will be
+ * discarded 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 extend the tracked request for this
+ * zero write until INT64_MAX (effectively infinity), and mark
+ * it as serializing.
+ *
+ * We have to enable this workaround for all filesystems and
+ * AIO modes (not just XFS with aio=native), because for
+ * remote filesystems we do not know the host configuration.
+ * However, this should have no effect as long as no block
+ * driver submits zero writes beyond the EOF.
+ */
+
+ req = bdrv_co_get_self_request(bs);
+ assert(req);
+ assert(req->type == BDRV_TRACKED_WRITE);
+ 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
