linux-kernel - Re: [PATCH] mm: workingset: fix NULL ptr dereference

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Date:   Mon, 9 Apr 2018 17:34:09 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.org>, Chao Yu <yuchao0@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        Chris Fries <cfries@...gle.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mm: workingset: fix NULL ptr dereference

On Mon, Apr 09, 2018 at 03:52:15PM +0200, Michal Hocko wrote:
> On Mon 09-04-18 06:41:14, Matthew Wilcox wrote:
> > On Mon, Apr 09, 2018 at 02:48:52PM +0200, Michal Hocko wrote:
> > > On Mon 09-04-18 20:25:06, Chao Yu wrote:
> > > [...]
> > > > diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
> > > > index c85cccc2e800..cc63f8c448f0 100644
> > > > --- a/fs/f2fs/inode.c
> > > > +++ b/fs/f2fs/inode.c
> > > > @@ -339,10 +339,10 @@ struct inode *f2fs_iget(struct super_block *sb, unsigned long ino)
> > > >  make_now:
> > > >  	if (ino == F2FS_NODE_INO(sbi)) {
> > > >  		inode->i_mapping->a_ops = &f2fs_node_aops;
> > > > -		mapping_set_gfp_mask(inode->i_mapping, GFP_F2FS_ZERO);
> > > > +		mapping_set_gfp_mask(inode->i_mapping, GFP_NOFS);
> > > 
> > > An unrelated question. Why do you make all allocations for the mapping
> > > NOFS automatically? What kind of reclaim recursion problems are you
> > > trying to prevent?
> > 
> > It's worth noting that this is endemic in filesystems.
> 
> Yes, and I have strong suspicion that this is a mindless copy&pasting...
> Well, xfs had a good reason for it in the past - mostly to handle deep
> call stacks on complicated storage setups in the past when we used to
> trigger IO from the direct reclaim. I am not sure whether there are
> other reasons to keep the status quo except for finding somebody brave
> enough to post the patch, do all the due testing.
> 
> > $ git grep mapping_set_gfp_mask.*FS
> > drivers/block/loop.c:   mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));
> > fs/btrfs/disk-io.c:     mapping_set_gfp_mask(fs_info->btree_inode->i_mapping, GFP_NOFS);

Thi was added in 1561deda687eef0
(https://git.kernel.org/linus/1561deda687eef0e9506) and probably after a
deadlock report.

The changelog mentions the potential recursion from fs -> allocation -> fs,
but I'm not sure if this still happens on the MM side today.

For the filesystem part, I think the key functions of the callchain are
still there.

The code was been added in 2011 and the 2nd hunk of the patch added a
code that's not present today AFAICS, so this is worth revisiting.

I still don't understand how it's related to the GFP_HIGHUSER_MOVABLE,
this patch is from time where the metadata pages were possibly allocated
from HIGHMEM but this was removed later in a65917156e345946db
(https://git.kernel.org/linus/a65917156e345946db).