[PATCH v2] xfs: don't zero partial page cache pages during O_DIRECT

[Brian Foster]

On Sat, Aug 09, 2014 at 08:57:00AM -0400, Chris Mason wrote:
> 
> xfs is using truncate_pagecache_range to invalidate the page cache
> during DIO reads.  This is different from the other filesystems who only
> invalidate pages during DIO writes.
> 
> truncate_pagecache_range is meant to be used when we are freeing the
> underlying data structs from disk, so it will zero any partial ranges in
> the page.  This means a DIO read can zero out part of the page cache
> page, and it is possible the page will stay in cache.
> 
> buffered reads will find an up to date page with zeros instead of the
> data actually on disk.
> 
> This patch fixes things by using invalidate_inode_pages2_range instead.
> It preserves the page cache invalidation, but won't zero any pages.
> 
> Signed-off-by: Chris Mason <clm at fb.com>
> cc: stable at vger.kernel.org
> 
> ---

I can't say I grok the risk of removing the invalidation entirely, but
this is certainly a more isolated fix:

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

>  fs/xfs/xfs_file.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 1f66779..023d575 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -295,7 +295,8 @@ xfs_file_read_iter(
>  				xfs_rw_iunlock(ip, XFS_IOLOCK_EXCL);
>  				return ret;
>  			}
> -			truncate_pagecache_range(VFS_I(ip), pos, -1);
> +			invalidate_inode_pages2_range(VFS_I(ip)->i_mapping,
> +					      pos >> PAGE_CACHE_SHIFT, -1);
>  		}
>  		xfs_rw_ilock_demote(ip, XFS_IOLOCK_EXCL);
>  	}
> -- 
> 1.8.1
> 
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