file-posix: Make bdrv_flush() failure permanent without O_DIRECT
Success for bdrv_flush() means that all previously written data is safe
on disk. For fdatasync(), the best semantics we can hope for on Linux
(without O_DIRECT) is that all data that was written since the last call
was successfully written back. Therefore, and because we can't redo all
writes after a flush failure, we have to give up after a single
fdatasync() failure. After this failure, we would never be able to make
the promise that a successful bdrv_flush() makes.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20170322210005.16533-1-kwolf@redhat.com
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/block/file-posix.c b/block/file-posix.c
index 53febd3..beb7a4f 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -144,6 +144,7 @@
bool has_write_zeroes:1;
bool discard_zeroes:1;
bool use_linux_aio:1;
+ bool page_cache_inconsistent:1;
bool has_fallocate;
bool needs_alignment;
} BDRVRawState;
@@ -824,10 +825,31 @@
static ssize_t handle_aiocb_flush(RawPosixAIOData *aiocb)
{
+ BDRVRawState *s = aiocb->bs->opaque;
int ret;
+ if (s->page_cache_inconsistent) {
+ return -EIO;
+ }
+
ret = qemu_fdatasync(aiocb->aio_fildes);
if (ret == -1) {
+ /* There is no clear definition of the semantics of a failing fsync(),
+ * so we may have to assume the worst. The sad truth is that this
+ * assumption is correct for Linux. Some pages are now probably marked
+ * clean in the page cache even though they are inconsistent with the
+ * on-disk contents. The next fdatasync() call would succeed, but no
+ * further writeback attempt will be made. We can't get back to a state
+ * in which we know what is on disk (we would have to rewrite
+ * everything that was touched since the last fdatasync() at least), so
+ * make bdrv_flush() fail permanently. Given that the behaviour isn't
+ * really defined, I have little hope that other OSes are doing better.
+ *
+ * Obviously, this doesn't affect O_DIRECT, which bypasses the page
+ * cache. */
+ if ((s->open_flags & O_DIRECT) == 0) {
+ s->page_cache_inconsistent = true;
+ }
return -errno;
}
return 0;
