From: Jan Kara <jack@suse.cz>
Subject: Re: [PATCH 8/8] Revert "ext4: fix wrong gfp type under transaction"
Date: Tue, 17 Jan 2017 18:29:25 +0100
Message-ID: <20170117172925.GA2486@quack2.suse.cz>
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Cc: Theodore Ts'o <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Chinner <david@fromorbit.com>, djwong@kernel.org,
	Chris Mason <clm@fb.com>, David Sterba <dsterba@suse.cz>,
	ceph-devel@vger.kernel.org, cluster-devel@redhat.com,
	linux-nfs@vger.kernel.org, logfs@logfs.org,
	linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-btrfs@vger.kernel.org, linux-mtd@lists.infradead.org,
	reiserfs-devel@vger.kernel.org,
	linux-ntfs-dev@lists.sourceforge.net,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-afs@lists.infradead.org, LKML <linux-kernel@vger.kernel.org>
To: Michal Hocko <mhocko@kernel.org>
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On Tue 17-01-17 17:16:19, Michal Hocko wrote:
> > > But before going to play with that I am really wondering whether we need
> > > all this with no journal at all. AFAIU what Jack told me it is the
> > > journal lock(s) which is the biggest problem from the reclaim recursion
> > > point of view. What would cause a deadlock in no journal mode?
> > 
> > We still have the original problem for why we need GFP_NOFS even in
> > ext2.  If we are in a writeback path, and we need to allocate memory,
> > we don't want to recurse back into the file system's writeback path.
> 
> But we do not enter the writeback path from the direct reclaim. Or do
> you mean something other than pageout()'s mapping->a_ops->writepage?
> There is only try_to_release_page where we get back to the filesystems
> but I do not see any NOFS protection in ext4_releasepage.

Maybe to expand a bit: These days, direct reclaim can call ->releasepage()
callback, ->evict_inode() callback (and only for inodes with i_nlink > 0),
shrinkers. That's it. So the recursion possibilities are rather more limited
than they used to be several years ago and we likely do not need as much
GFP_NOFS protection as we used to.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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