Re: [PATCH] xfs: use MMAPLOCK around filemap_map_pages()

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Jun 23, 2020 at 03:20:59PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> The page faultround path ->map_pages is implemented in XFS via
> filemap_map_pages(). This function checks that pages found in page
> cache lookups have not raced with truncate based invalidation by
> checking page->mapping is correct and page->index is within EOF.
> 
> However, we've known for a long time that this is not sufficient to
> protect against races with invalidations done by operations that do
> not change EOF. e.g. hole punching and other fallocate() based
> direct extent manipulations. The way we protect against these
> races is we wrap the page fault operations in a XFS_MMAPLOCK_SHARED
> lock so they serialise against fallocate and truncate before calling
> into the filemap function that processes the fault.
> 
> Do the same for XFS's ->map_pages implementation to close this
> potential data corruption issue.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Looks ok, notwithstanding the (semirelated) questions that Amir
unearthed elsewhere in this thread...

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  fs/xfs/xfs_file.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 7b05f8fd7b3d..4b185a907432 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1266,10 +1266,23 @@ xfs_filemap_pfn_mkwrite(
>  	return __xfs_filemap_fault(vmf, PE_SIZE_PTE, true);
>  }
>  
> +static void
> +xfs_filemap_map_pages(
> +	struct vm_fault		*vmf,
> +	pgoff_t			start_pgoff,
> +	pgoff_t			end_pgoff)
> +{
> +	struct inode		*inode = file_inode(vmf->vma->vm_file);
> +
> +	xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> +	filemap_map_pages(vmf, start_pgoff, end_pgoff);
> +	xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> +}
> +
>  static const struct vm_operations_struct xfs_file_vm_ops = {
>  	.fault		= xfs_filemap_fault,
>  	.huge_fault	= xfs_filemap_huge_fault,
> -	.map_pages	= filemap_map_pages,
> +	.map_pages	= xfs_filemap_map_pages,
>  	.page_mkwrite	= xfs_filemap_page_mkwrite,
>  	.pfn_mkwrite	= xfs_filemap_pfn_mkwrite,
>  };
> -- 
> 2.26.2.761.g0e0b3e54be
>