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

[Yafang Shao <laoar.shao@gmail.com>]

On Mon, Jun 15, 2020 at 10:53 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> 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.

Hi Michal,

My understanding is that this warning is to tell us we don't want a
recursive filesystem reclaim with PF_MEMALLOC_NOFS being specifically
set, but unfortunately PF_MEMALLOC_NOFS doesn't work so it comes here
again.

IOW, PF_MEMALLOC_NOFS can be set after this check, like what I did in v2. [1]


> 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.

Thanks for your information.
As pointed out by Dave in v1[2] that iomap_do_writepage() can be
called with a locked page
cache page and calls ->map_blocks from that context.

[1]. https://lore.kernel.org/linux-xfs/1591254347-15912-1-git-send-email-laoar.shao@gmail.com/
[2] https://lore.kernel.org/linux-xfs/20200603222741.GQ2040@dread.disaster.area/



-- 
Thanks
Yafang