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

To: NeilBrown <neilb@xxxxxxx>
Subject: Re: [PATCH 09/19] XFS: ensure xfs_file_*_read cannot deadlock in memory allocation.
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 16 Apr 2014 16:04:59 +1000
Cc: linux-mm@xxxxxxxxx, linux-nfs@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140416040336.10604.90380.stgit@xxxxxxxxxxxxxx>
References: <20140416033623.10604.69237.stgit@xxxxxxxxxxxxxx> <20140416040336.10604.90380.stgit@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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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