Re: [PATCH] mm: workingset: fix NULL ptr dereference

From: Michal Hocko <mhocko@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Chao Yu <yuchao0@huawei.com>, Minchan Kim <minchan@kernel.org>,
	Jaegeuk Kim <jaegeuk@kernel.org>,
	Christopher Lameter <cl@linux.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>, Jan Kara <jack@suse.cz>,
	Chris Fries <cfries@google.com>,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] mm: workingset: fix NULL ptr dereference
Date: Mon, 9 Apr 2018 15:52:15 +0200	[thread overview]
Message-ID: <20180409135215.GH21835@dhcp22.suse.cz> (raw)
In-Reply-To: <20180409134114.GA30963@bombadil.infradead.org>

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);
> fs/f2fs/inode.c:                mapping_set_gfp_mask(inode->i_mapping, GFP_F2FS_ZERO);
> fs/f2fs/inode.c:                mapping_set_gfp_mask(inode->i_mapping, GFP_F2FS_ZERO);
> fs/gfs2/glock.c:                mapping_set_gfp_mask(mapping, GFP_NOFS);
> fs/gfs2/ops_fstype.c:   mapping_set_gfp_mask(mapping, GFP_NOFS);
> fs/jfs/jfs_imap.c:      mapping_set_gfp_mask(ip->i_mapping, GFP_NOFS);
> fs/jfs/super.c: mapping_set_gfp_mask(inode->i_mapping, GFP_NOFS);
> fs/nilfs2/gcinode.c:    mapping_set_gfp_mask(inode->i_mapping, GFP_NOFS);
> fs/nilfs2/page.c:       mapping_set_gfp_mask(mapping, GFP_NOFS);
> fs/reiserfs/xattr.c:    mapping_set_gfp_mask(mapping, GFP_NOFS);
> fs/xfs/xfs_iops.c:      mapping_set_gfp_mask(inode->i_mapping, (gfp_mask & ~(__GFP_FS)));

-- 
Michal Hocko
SUSE Labs