Re: [PATCH V4 09/13] fs/xfs: Add write aops lock to xfs layer

[Dave Chinner]

On Thu, Feb 20, 2020 at 04:41:30PM -0800, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> XFS requires the use of the aops of an inode to quiesced prior to
> changing it to/from the DAX aops vector.
> 
> Take the aops write lock while changing DAX state.
> 
> We define a new XFS_DAX_EXCL lock type to carry the lock through to
> transaction completion.
> 
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> ---
> Changes from v3:
> 	Change locking function names to reflect changes in previous
> 	patches.
> 
> Changes from V2:
> 	Change name of patch (WAS: fs/xfs: Add lock/unlock state to xfs)
> 	Remove the xfs specific lock and move to the vfs layer.
> 		We still use XFS_LOCK_DAX_EXCL to be able to pass this
> 		flag through to the transaction code.  But we no longer
> 		have a lock specific to xfs.  This removes a lot of code
> 		from the XFS layer, preps us for using this in ext4, and
> 		is actually more straight forward now that all the
> 		locking requirements are better known.
> 
> 	Fix locking order comment
> 	Rework for new 'state' names
> 	(Other comments on the previous patch are not applicable with
> 	new patch as much of the code was removed in favor of the vfs
> 	level lock)
> ---
>  fs/xfs/xfs_inode.c | 22 ++++++++++++++++++++--
>  fs/xfs/xfs_inode.h |  7 +++++--
>  2 files changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 35df324875db..5b014c428f0f 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -142,12 +142,12 @@ xfs_ilock_attr_map_shared(
>   *
>   * Basic locking order:
>   *
> - * i_rwsem -> i_mmap_lock -> page_lock -> i_ilock
> + * s_dax_sem -> i_rwsem -> i_mmap_lock -> page_lock -> i_ilock
>   *
>   * mmap_sem locking order:
>   *
>   * i_rwsem -> page lock -> mmap_sem
> - * mmap_sem -> i_mmap_lock -> page_lock
> + * s_dax_sem -> mmap_sem -> i_mmap_lock -> page_lock
>   *
>   * The difference in mmap_sem locking order mean that we cannot hold the
>   * i_mmap_lock over syscall based read(2)/write(2) based IO. These IO paths can
> @@ -182,6 +182,9 @@ xfs_ilock(
>  	       (XFS_ILOCK_SHARED | XFS_ILOCK_EXCL));
>  	ASSERT((lock_flags & ~(XFS_LOCK_MASK | XFS_LOCK_SUBCLASS_MASK)) == 0);
>  
> +	if (lock_flags & XFS_DAX_EXCL)
> +		inode_aops_down_write(VFS_I(ip));

I largely don't see the point of adding this to xfs_ilock/iunlock.

It's only got one caller, so I don't see much point in adding it to
an interface that has over a hundred other call sites that don't
need or use this lock. just open code it where it is needed in the
ioctl code.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx