Re: [PATCH 6/6] xfs: xfs_filemap_pmd_fault treats read faults as write faults

[Brian Foster <bfoster@redhat.com>]

On Mon, Oct 19, 2015 at 02:27:18PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> The code initially committed didn't have the same checks for write
> faults as the dax_pmd_fault code and hence treats all faults as
> write faults. We can get read faults through this path because they
> is no pmd_mkwrite path for write faults similar to the normal page
> fault path. Hence we need to ensure that we only do c/mtime updates
> on write faults, and freeze protection is unnecessary for read
> faults.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  fs/xfs/xfs_file.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index e7cf9ec..0045b0a 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1482,7 +1482,7 @@ xfs_file_llseek(
>   *
>   * mmap_sem (MM)
>   *   sb_start_pagefault(vfs, freeze)
> - *     i_mmap_lock (XFS - truncate serialisation)
> + *     i_mmaplock (XFS - truncate serialisation)
>   *       page_lock (MM)
>   *         i_lock (XFS - extent map serialisation)
>   */
> @@ -1550,6 +1550,13 @@ xfs_filemap_fault(
>  	return ret;
>  }
>  
> +/*
> + * Similar to xfs_filemap_fault(), the DAX fault path can call into here on
> + * both read and write faults. Hence we need to handle both cases. There is no
> + * ->pmd_mkwrite callout for huge pages, so we have a single function here to
> + * handle both cases here. @flags carries the information on the type of fault
> + * occuring.
> + */
>  STATIC int
>  xfs_filemap_pmd_fault(
>  	struct vm_area_struct	*vma,
> @@ -1566,13 +1573,18 @@ xfs_filemap_pmd_fault(
>  
>  	trace_xfs_filemap_pmd_fault(ip);
>  
> -	sb_start_pagefault(inode->i_sb);
> -	file_update_time(vma->vm_file);
> +	if (flags & FAULT_FLAG_WRITE) {
> +		sb_start_pagefault(inode->i_sb);
> +		file_update_time(vma->vm_file);
> +	}
> +
>  	xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
>  	ret = __dax_pmd_fault(vma, addr, pmd, flags, xfs_get_blocks_dax_fault,
>  			      NULL);
>  	xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> -	sb_end_pagefault(inode->i_sb);
> +
> +	if (flags & FAULT_FLAG_WRITE)
> +		sb_end_pagefault(inode->i_sb);
>  
>  	return ret;
>  }
> -- 
> 2.5.0
> 
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