Re: [PATCH 09/19] XFS: ensure xfs_file_*_read cannot deadlock in memory allocation.

[Dave Chinner]

On Wed, Apr 16, 2014 at 02:03:36PM +1000, NeilBrown wrote:
> xfs_file_*_read holds an inode lock while calling a generic 'read'
> function.  These functions perform read-ahead and are quite likely to
> allocate memory.

Yes, that's what reading data from disk requires.

> So set PF_FSTRANS to ensure they avoid __GFP_FS and so don't recurse
> into a filesystem to free memory.

We already have that protection via the
> 
> This can be a problem with loop-back NFS mounts, if free_pages ends up
> wating in nfs_release_page(), and nfsd is blocked waiting for the lock
> that this code holds.
> 
> This was found both by lockdep and as a real deadlock during testing.
> 
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
>  fs/xfs/xfs_file.c |   12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 64b48eade91d..88b33ef64668 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -243,6 +243,7 @@ xfs_file_aio_read(
>  	ssize_t			ret = 0;
>  	int			ioflags = 0;
>  	xfs_fsize_t		n;
> +	unsigned int		pflags;
>  
>  	XFS_STATS_INC(xs_read_calls);
>  
> @@ -290,6 +291,10 @@ xfs_file_aio_read(
>  	 * proceeed concurrently without serialisation.
>  	 */
>  	xfs_rw_ilock(ip, XFS_IOLOCK_SHARED);
> +	/* As we hold a lock, we must ensure that any allocation
> +	 * in generic_file_aio_read avoid __GFP_FS
> +	 */
> +	current_set_flags_nested(&pflags, PF_FSTRANS);

Ugh. No. This is Simply Wrong.

We handle the memory allocations in the IO path with
GFP_NOFS/KM_NOFS where necessary.

We also do this when setting up regular file inodes in
xfs_setup_inode():

        /*
         * Ensure all page cache allocations are done from GFP_NOFS context to
         * prevent direct reclaim recursion back into the filesystem and blowing
         * stacks or deadlocking.
         */
        gfp_mask = mapping_gfp_mask(inode->i_mapping);
        mapping_set_gfp_mask(inode->i_mapping, (gfp_mask & ~(__GFP_FS)));

Which handles all of the mapping allocations that occur within the
page cache read/write paths.

Remember, you removed the KM_NOFS code from the XFS allocator that
caused it to clear __GFP_FS in an earlier patch - the read Io path
is one of the things you broke by doing that....

If there are places where we don't use GFP_NOFS context allocations
that we should, then we need to fix them individually....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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