linux-ext4 - Re: [PATCH 8/8] Revert "ext4: fix wrong gfp type under transaction"

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Message-ID: <20170126074455.GC8456@dhcp22.suse.cz>
Date:   Thu, 26 Jan 2017 08:44:55 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>, djwong@...nel.org,
        Chris Mason <clm@...com>, David Sterba <dsterba@...e.cz>,
        ceph-devel@...r.kernel.org, cluster-devel@...hat.com,
        linux-nfs@...r.kernel.org, logfs@...fs.org,
        linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-mtd@...ts.infradead.org,
        reiserfs-devel@...r.kernel.org,
        linux-ntfs-dev@...ts.sourceforge.net,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-afs@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 8/8] Revert "ext4: fix wrong gfp type under transaction"

On Thu 19-01-17 10:44:05, Michal Hocko wrote:
> On Thu 19-01-17 10:22:36, Jan Kara wrote:
> > On Thu 19-01-17 09:39:56, Michal Hocko wrote:
> > > On Tue 17-01-17 18:29:25, Jan Kara wrote:
> > > > 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.
> > > 
> > > Thanks for making my remark more clear Jack! I would just want to add
> > > that I was playing with the patch below (it is basically
> > > GFP_NOFS->GFP_KERNEL for all allocations which trigger warning from the
> > > debugging patch which means they are called from within transaction) and
> > > it didn't hit the lockdep when running xfstests both with or without the
> > > enabled journal.
> > > 
> > > So am I still missing something or the nojournal mode is safe and the
> > > current series is OK wrt. ext*?
> > 
> > I'm convinced the current series is OK, only real life will tell us whether
> > we missed something or not ;)
> 
> I would like to extend the changelog of "jbd2: mark the transaction
> context with the scope GFP_NOFS context".
> 
> "
> Please note that setups without journal do not suffer from potential
> recursion problems and so they do not need the scope protection because
> neither ->releasepage nor ->evict_inode (which are the only fs entry
> points from the direct reclaim) can reenter a locked context which is
> doing the allocation currently.
> "

Could you comment on this Ted, please?
-- 
Michal Hocko
SUSE Labs