ext4: Do not fallback to buffered-io for DIO atomic write

atomic writes is currently only supported for single fsblock and only
for direct-io. We should not return -ENOTBLK for atomic writes since we
want the atomic write request to either complete fully or fail
otherwise. Hence, we should never fallback to buffered-io in case of
DIO atomic write requests.
Let's also catch if this ever happens by adding some WARN_ON_ONCE before
buffered-io handling for direct-io atomic writes. More details of the
discussion [1].

While at it let's add an inline helper ext4_want_directio_fallback() which
simplifies the logic checks and inherently fixes condition on when to return
-ENOTBLK which otherwise was always returning true for any write or directio in
ext4_iomap_end(). It was ok since ext4 only supports direct-io via iomap.

[1]: https://lore.kernel.org/linux-xfs/cover.1729825985.git.ritesh.list@gmail.com/T/#m9dbecc11bed713ed0d7a486432c56b105b555f04
Suggested-by: Darrick J. Wong <djwong@kernel.org> # inline helper
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 96d936f5584bdbf48664e77847b75bccdce91d96..a7de03e47db082f016c579b8e93b65d6937401cd 100644 (file)
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -599,6 +599,13 @@ out:
                ssize_t err;
                loff_t endbyte;
 
+               /*
+                * There is no support for atomic writes on buffered-io yet,
+                * we should never fallback to buffered-io for DIO atomic
+                * writes.
+                */
+               WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC);
+
                offset = iocb->ki_pos;
                err = ext4_buffered_write_iter(iocb, from);
                if (err < 0)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 3e827cfa762e458239267ef4a4a5131484e84ac8..5b9eeb74ce476696acfbcc6113a0ada21b928c86 100644 (file)
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3444,17 +3444,34 @@ static int ext4_iomap_overwrite_begin(struct inode *inode, loff_t offset,
        return ret;
 }
 
+static inline bool ext4_want_directio_fallback(unsigned flags, ssize_t written)
+{
+       /* must be a directio to fall back to buffered */
+       if ((flags & (IOMAP_WRITE | IOMAP_DIRECT)) !=
+                   (IOMAP_WRITE | IOMAP_DIRECT))
+               return false;
+
+       /* atomic writes are all-or-nothing */
+       if (flags & IOMAP_ATOMIC)
+               return false;
+
+       /* can only try again if we wrote nothing */
+       return written == 0;
+}
+
 static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
                          ssize_t written, unsigned flags, struct iomap *iomap)
 {
        /*
         * Check to see whether an error occurred while writing out the data to
-        * the allocated blocks. If so, return the magic error code so that we
-        * fallback to buffered I/O and attempt to complete the remainder of
-        * the I/O. Any blocks that may have been allocated in preparation for
-        * the direct I/O will be reused during buffered I/O.
+        * the allocated blocks. If so, return the magic error code for
+        * non-atomic write so that we fallback to buffered I/O and attempt to
+        * complete the remainder of the I/O.
+        * For non-atomic writes, any blocks that may have been
+        * allocated in preparation for the direct I/O will be reused during
+        * buffered I/O. For atomic write, we never fallback to buffered-io.
         */
-       if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
+       if (ext4_want_directio_fallback(flags, written))
                return -ENOTBLK;
 
        return 0;