Re: [PATCH 2/2] xfs: xfs_reflink_convert_cow() memory allocation deadlock

[Dave Chinner <david@fromorbit.com>]

On Thu, Jun 07, 2018 at 07:41:40AM -0400, Brian Foster wrote:
> On Thu, Jun 07, 2018 at 03:21:32PM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > xfs_reflink_convert_cow() manipulates the incore extent list
> > in GFP_KERNEL context in the IO submission path whilst holding
> > locked pages under writeback. This is a memory reclaim deadlock
> > vector. This code is not in a transaction, so any memory allocations
> > it makes aren't protected via the memalloc_nofs_save() context that
> > transactions carry.
> > 
> > Hence we need to run this call under memalloc_nofs_save() context to
> > prevent potential memory allocations from being run as GFP_KERNEL
> > and deadlocking.
> > 
> > Signed-Off-By: Dave Chinner <dchinner@redhat.com>
> > ---
> 
> Looks fine modulo the header thing Allison already pointed out:
> 
> Reviewed-by: Brian Foster <bfoster@redhat.com>
> 
> BTW, shouldn't we also be using XFS_TRANS_NOFS in
> xfs_iomap_write_allocate()?

Most likely, yes.

However, I think the whole ->writepages path should be moved under
memalloc_nofs_save() context. I'm kinda waiting for the bufferhead
removal to land before auditing it fully and determining the scope
we should be covering by nofs alloc contexts. This was just a
drive-by patch to fix a problem that was reported...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com