Re: [PATCH v3] xfs: avoid deadlock when trigger memory reclaim in ->writepages

[Michal Hocko <mhocko@kernel.org>]

On Mon 15-06-20 16:25:52, Holger Hoffstätte wrote:
> On 2020-06-15 13:56, Yafang Shao wrote:
[...]
> > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > index b356118..1ccfbf2 100644
> > --- a/fs/xfs/xfs_aops.c
> > +++ b/fs/xfs/xfs_aops.c
> > @@ -573,9 +573,21 @@ static inline bool xfs_ioend_needs_workqueue(struct iomap_ioend *ioend)
> >   	struct writeback_control *wbc)
> >   {
> >   	struct xfs_writepage_ctx wpc = { };
> > +	unsigned int nofs_flag;
> > +	int ret;
> >   	xfs_iflags_clear(XFS_I(mapping->host), XFS_ITRUNCATED);
> > -	return iomap_writepages(mapping, wbc, &wpc.ctx, &xfs_writeback_ops);
> > +
> > +	/*
> > +	 * We can allocate memory here while doing writeback on behalf of
> > +	 * memory reclaim.  To avoid memory allocation deadlocks set the
> > +	 * task-wide nofs context for the following operations.
> > +	 */
> > +	nofs_flag = memalloc_nofs_save();
> > +	ret = iomap_writepages(mapping, wbc, &wpc.ctx, &xfs_writeback_ops);
> > +	memalloc_nofs_restore(nofs_flag);
> > +
> > +	return ret;
> >   }
> >   STATIC int
> > 
> 
> Not sure if I did something wrong, but while the previous version of this patch
> worked fine, this one gave me (with v2 removed obviously):
> 
> [  +0.000004] WARNING: CPU: 0 PID: 2811 at fs/iomap/buffered-io.c:1544 iomap_do_writepage+0x6b4/0x780

This corresponds to
        /*
         * Given that we do not allow direct reclaim to call us, we should
         * never be called in a recursive filesystem reclaim context.
         */
        if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
                goto redirty;

which effectivelly says that memalloc_nofs_save/restore cannot be used
for that code path. Your stack trace doesn't point to a reclaim path
which shows that this path is shared and also underlines that this is
not really an intended use of the api. Please refer to
Documentation/core-api/gfp_mask-from-fs-io.rst for more details but
shortly the API should be used at the layer which defines a context
which shouldn't allow to recurse. E.g. a lock which would be problematic
in the reclaim recursion path.
-- 
Michal Hocko
SUSE Labs