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

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Date:   Fri, 3 Feb 2017 16:32:22 +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 Mon 30-01-17 09:12:10, Michal Hocko wrote:
> 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.

ping on this. I would really like to move forward here and target 4.11
merge window. Is your concern so serious to block this patch?
-- 
Michal Hocko
SUSE Labs