On Thu, Jul 17, 2025 at 03:19:00PM +0000, John Garry wrote:
> Atomic writes are not currently supported for DAX, but two problems exist:
> - we may go down DAX write path for IOCB_ATOMIC, which does not handle
> IOCB_ATOMIC properly
> - we report non-zero atomic write limits in statx (for DAX inodes)
>
> We may want atomic writes support on DAX in future, but just disallow for
> now.
>
> For this, ensure when IOCB_ATOMIC is set that we check the write size
> versus the atomic write min and max before branching off to the DAX write
> path. This is not strictly required for DAX, as we should not get this far
> in the write path as FMODE_CAN_ATOMIC_WRITE should not be set.
>
> In addition, due to reflink being supported for DAX, we automatically get
> CoW-based atomic writes support being advertised. Remedy this by
> disallowing atomic writes for a DAX inode for both sw and hw modes.
...because it's fsdax and who's really going to test/use software atomic
writes there ?
> Finally make atomic write size and DAX mount always mutually exclusive.
Why? You could have a rt-on-pmem filesystem with S_DAX files, and still
want to do atomic writes to files on the data device.
> Reported-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> Fixes: 9dffc58f2384 ("xfs: update atomic write limits")
> Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
>
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index db21b5a4b881..84876f41da93 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1102,9 +1102,6 @@ xfs_file_write_iter(
> if (xfs_is_shutdown(ip->i_mount))
> return -EIO;
>
> - if (IS_DAX(inode))
> - return xfs_file_dax_write(iocb, from);
> -
> if (iocb->ki_flags & IOCB_ATOMIC) {
> if (ocount < xfs_get_atomic_write_min(ip))
> return -EINVAL;
> @@ -1117,6 +1114,9 @@ xfs_file_write_iter(
> return ret;
> }
>
> + if (IS_DAX(inode))
> + return xfs_file_dax_write(iocb, from);
> +
> if (iocb->ki_flags & IOCB_DIRECT) {
> /*
> * Allow a directio write to fall back to a buffered
> diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> index 07fbdcc4cbf5..b142cd4f446a 100644
> --- a/fs/xfs/xfs_inode.h
> +++ b/fs/xfs/xfs_inode.h
> @@ -356,11 +356,22 @@ static inline bool xfs_inode_has_bigrtalloc(const struct xfs_inode *ip)
> (XFS_IS_REALTIME_INODE(ip) ? \
> (ip)->i_mount->m_rtdev_targp : (ip)->i_mount->m_ddev_targp)
>
> -static inline bool xfs_inode_can_hw_atomic_write(const struct xfs_inode *ip)
> +static inline bool xfs_inode_can_hw_atomic_write(struct xfs_inode *ip)
Why drop const here? VFS_IC() should be sufficient, I think.
--D
> {
> + if (IS_DAX(VFS_I(ip)))
> + return false;
> +
> return xfs_inode_buftarg(ip)->bt_awu_max > 0;
> }
>
> +static inline bool xfs_inode_can_sw_atomic_write(struct xfs_inode *ip)
> +{
> + if (IS_DAX(VFS_I(ip)))
> + return false;
> +
> + return xfs_can_sw_atomic_write(ip->i_mount);
> +}
> +
> /*
> * In-core inode flags.
> */
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 01e597290eb5..b39a19dbc386 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -616,7 +616,8 @@ xfs_get_atomic_write_min(
> * write of exactly one single fsblock if the bdev will make that
> * guarantee for us.
> */
> - if (xfs_inode_can_hw_atomic_write(ip) || xfs_can_sw_atomic_write(mp))
> + if (xfs_inode_can_hw_atomic_write(ip) ||
> + xfs_inode_can_sw_atomic_write(ip))
> return mp->m_sb.sb_blocksize;
>
> return 0;
> @@ -633,7 +634,7 @@ xfs_get_atomic_write_max(
> * write of exactly one single fsblock if the bdev will make that
> * guarantee for us.
> */
> - if (!xfs_can_sw_atomic_write(mp)) {
> + if (!xfs_inode_can_sw_atomic_write(ip)) {
> if (xfs_inode_can_hw_atomic_write(ip))
> return mp->m_sb.sb_blocksize;
> return 0;
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index 0b690bc119d7..6a5543e08198 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -745,6 +745,12 @@ xfs_set_max_atomic_write_opt(
>
> ASSERT(max_write <= U32_MAX);
>
> + if (xfs_has_dax_always(mp)) {
> + xfs_warn(mp,
> + "atomic writes not supported for DAX");
> + return -EINVAL;
> + }
> +
> /* generic_atomic_write_valid enforces power of two length */
> if (!is_power_of_2(new_max_bytes)) {
> xfs_warn(mp,
> diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> index 97de44c32272..3c858b42a54a 100644
> --- a/fs/xfs/xfs_mount.h
> +++ b/fs/xfs/xfs_mount.h
> @@ -474,11 +474,6 @@ static inline bool xfs_has_nonzoned(const struct xfs_mount *mp)
> return !xfs_has_zoned(mp);
> }
>
> -static inline bool xfs_can_sw_atomic_write(struct xfs_mount *mp)
> -{
> - return xfs_has_reflink(mp);
> -}
> -
> /*
> * Some features are always on for v5 file systems, allow the compiler to
> * eliminiate dead code when building without v4 support.
> @@ -534,6 +529,14 @@ __XFS_HAS_FEAT(dax_never, DAX_NEVER)
> __XFS_HAS_FEAT(norecovery, NORECOVERY)
> __XFS_HAS_FEAT(nouuid, NOUUID)
>
> +static inline bool xfs_can_sw_atomic_write(struct xfs_mount *mp)
> +{
> + if (xfs_has_dax_always(mp))
> + return false;
> +
> + return xfs_has_reflink(mp);
> +}
> +
> /*
> * Operational mount state flags
> *
> --
> 2.43.5
>
>
