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

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Message-ID: <20170130081210.GD8443@dhcp22.suse.cz>
Date:   Mon, 30 Jan 2017 09:12:10 +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 Fri 27-01-17 11:40:42, Theodore Ts'o wrote:
> On Fri, Jan 27, 2017 at 10:37:35AM +0100, Michal Hocko wrote:
> > If this ever turn out to be a problem and with the vmapped stacks we
> > have good chances to get a proper stack traces on a potential overflow
> > we can add the scope API around the problematic code path with the
> > explanation why it is needed.
> 
> Yeah, or maybe we can automate it?  Can the reclaim code check how
> much stack space is left and do the right thing automatically?

I am not sure how to do that. Checking for some magic value sounds quite
fragile to me. It also sounds a bit strange to focus only on the reclaim
while other code paths might suffer from the same problem.

What is actually the deepest possible call chain from the slab reclaim
where I stopped? I have tried to follow that path but hit the callback
wall quite early.
 
> The reason why I'm nervous is that nojournal mode is not a common
> configuration, and "wait until production systems start failing" is
> not a strategy that I or many SRE-types find.... comforting.

I understand that but I would be much more happier if we did the
decision based on the actual data rather than a fear something would
break down.

-- 
Michal Hocko
SUSE Labs