[PATCH] file-posix: add statx(STATX_DIOALIGN) support

[Stefan Hajnoczi <stefanha@redhat.com>]

Linux v6.1 commit 825cf206ed51 ("statx: add direct I/O alignment
information") added an interface to fetch O_DIRECT alignment values for
block devices and file systems.

Prefer STATX_DIOALIGN to older interfaces and probing, but keep them as
fallbacks in case STATX_DIOALIGN cannot provide the information.

STATX_DIOALIGN support as of Linux 6.1:
- btrfs: no
- ext4: yes
- XFS: yes
- block devices: yes
- dm-crypt: yes

This change fixes #1290 ("IO alignment probing delivers incorrect
results on Linux when used with e.g. dm-crypt") on Linux host kernel
version 6.1 or later by avoiding the problematic probing code in
raw_probe_alignment(). It would be nice to fix the probing code but I
have not been able to reproduce it. Stable/LTS distros ship Linux 6.1+,
so the number of users without STATX_DIOALIGN is becoming small.

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/1290
Cc: Eric Biggers <ebiggers@google.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
v3:
- Rebase and update commit description
v2:
- Make sure that XFS_IOC_DIOINFO takes priority over logical blocksize [Eric Biggers]
---
 block/file-posix.c | 56 +++++++++++++++++++++++++++++++---------------
 1 file changed, 38 insertions(+), 18 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 9aad156aad4..0dedd090068 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -426,28 +426,48 @@ static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp)
 
     bs->bl.request_alignment = 0;
     s->buf_align = 0;
-    /* Let's try to use the logical blocksize for the alignment. */
-    if (probe_logical_blocksize(fd, &bs->bl.request_alignment) < 0) {
-        bs->bl.request_alignment = 0;
+
+#if defined(__linux__) && defined(STATX_DIOALIGN)
+    struct statx stx;
+
+    /*
+     * Linux 6.1 introduced an interface for both block devices and file
+     * systems. The system call returns with the STATX_DIOALIGN bit cleared
+     * when the information is unavailable.
+     */
+    if (statx(fd, "", AT_EMPTY_PATH, STATX_DIOALIGN, &stx) == 0 &&
+        (stx.stx_mask & STATX_DIOALIGN)) {
+        bs->bl.request_alignment = stx.stx_dio_offset_align;
+        s->buf_align = stx.stx_dio_mem_align;
     }
+#endif /* defined(__linux__) && defined(STATX_DIOALIGN) */
 
 #ifdef __linux__
-    /*
-     * The XFS ioctl definitions are shipped in extra packages that might
-     * not always be available. Since we just need the XFS_IOC_DIOINFO ioctl
-     * here, we simply use our own definition instead:
-     */
-    struct xfs_dioattr {
-        uint32_t d_mem;
-        uint32_t d_miniosz;
-        uint32_t d_maxiosz;
-    } da;
-    if (ioctl(fd, _IOR('X', 30, struct xfs_dioattr), &da) >= 0) {
-        bs->bl.request_alignment = da.d_miniosz;
-        /* The kernel returns wrong information for d_mem */
-        /* s->buf_align = da.d_mem; */
+    if (!bs->bl.request_alignment) {
+        /*
+         * The XFS ioctl definitions are shipped in extra packages that might
+         * not always be available. Since we just need the XFS_IOC_DIOINFO ioctl
+         * here, we simply use our own definition instead:
+         */
+        struct xfs_dioattr {
+            uint32_t d_mem;
+            uint32_t d_miniosz;
+            uint32_t d_maxiosz;
+        } da;
+        if (ioctl(fd, _IOR('X', 30, struct xfs_dioattr), &da) >= 0) {
+            bs->bl.request_alignment = da.d_miniosz;
+            /* The kernel returns wrong information for d_mem */
+            /* s->buf_align = da.d_mem; */
+        }
+    }
+#endif /* __linux__ */
+
+    /* Let's try to use the logical blocksize for the alignment. */
+    if (!bs->bl.request_alignment) {
+        if (probe_logical_blocksize(fd, &bs->bl.request_alignment) < 0) {
+            bs->bl.request_alignment = 0;
+        }
     }
-#endif
 
     /*
      * If we could not get the sizes so far, we can only guess them. First try
-- 
2.55.0