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

[NeilBrown <neilb@suse.de>]

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On Wed, 16 Apr 2014 16:04:59 +1000 Dave Chinner <david@fromorbit.com> wrote:

> 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@suse.de>
> > ---
> >  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.

Thanks Dave.  Having fixed the KM_NOFS error I'll start retesting with all
the other xfs patches removed.

NeilBrown

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