Linux dm-crypt returns errno EIO from unaligned O_DIRECT pread(2) calls.
Alignment probing fails on dm-crypt devices because the code expects
EINVAL.
Treating any errno as an "unaligned" indicator would be easy, but breaks
commit 22d182e82b4b ("block/raw-posix: fix launching with failed
disks"). Offline disks return EIO for correctly aligned requests and
EINVAL for unaligned requests.
It's possible to make both dm-crypt and offline disks work: look for the
transition from EINVAL to EIO instead of for a single EINVAL value.
Buglink: https://gitlab.com/qemu-project/qemu/-/issues/1290
Fixes: 22d182e82b4b ("block/raw-posix: fix launching with failed disks")
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
---
block/file-posix.c | 42 +++++++++++++++---------------------------
1 file changed, 15 insertions(+), 27 deletions(-)
diff --git a/block/file-posix.c b/block/file-posix.c
index b9647c5ffc..b9d62f52fe 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -355,31 +355,6 @@ static bool raw_needs_alignment(BlockDriverState *bs)
return s->force_alignment;
}
-/* Check if read is allowed with given memory buffer and length.
- *
- * This function is used to check O_DIRECT memory buffer and request alignment.
- */
-static bool raw_is_io_aligned(int fd, void *buf, size_t len)
-{
- ssize_t ret = pread(fd, buf, len, 0);
-
- if (ret >= 0) {
- return true;
- }
-
-#ifdef __linux__
- /* The Linux kernel returns EINVAL for misaligned O_DIRECT reads. Ignore
- * other errors (e.g. real I/O error), which could happen on a failed
- * drive, since we only care about probing alignment.
- */
- if (errno != EINVAL) {
- return true;
- }
-#endif
-
- return false;
-}
-
static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp)
{
BDRVRawState *s = bs->opaque;
@@ -426,34 +401,47 @@ static void raw_probe_alignment(BlockDriverState *bs, int
fd, Error **errp)
* try to detect buf_align, which cannot be detected in some cases (e.g.
* Gluster). If buf_align cannot be detected, we fallback to the value of
* request_alignment.
+ *
+ * The probing loop keeps track of the last errno so that the alignment of
+ * offline disks can be probed. On Linux pread(2) returns with errno EINVAL
+ * for most file descriptors when O_DIRECT alignment constraints are unmet.
+ * Offline disks fail correctly aligned pread(2) with EIO. Therefore it's
+ * possible to detect alignment on offline disks by observing when the
+ * errno changes from EINVAL to something else.
*/
if (!bs->bl.request_alignment) {
+ int last_errno = 0;
int i;
size_t align;
buf = qemu_memalign(max_align, max_align);
for (i = 0; i < ARRAY_SIZE(alignments); i++) {
align = alignments[i];
- if (raw_is_io_aligned(fd, buf, align)) {
+ if (pread(fd, buf, align, 0) >= 0 ||
+ (errno != EINVAL && last_errno == EINVAL)) {
/* Fallback to safe value. */
bs->bl.request_alignment = (align != 1) ? align : max_align;
break;
}
+ last_errno = errno;
}
qemu_vfree(buf);
}
if (!s->buf_align) {
+ int last_errno = 0;
int i;
size_t align;
buf = qemu_memalign(max_align, 2 * max_align);
for (i = 0; i < ARRAY_SIZE(alignments); i++) {
align = alignments[i];
- if (raw_is_io_aligned(fd, buf + align, max_align)) {
+ if (pread(fd, buf + align, max_align, 0) >= 0 ||
+ (errno != EINVAL && last_errno == EINVAL)) {
/* Fallback to request_alignment. */
s->buf_align = (align != 1) ? align : bs->bl.request_alignment;
break;
}
+ last_errno = errno;
}
qemu_vfree(buf);
}
--
2.38.1
