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

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Date:   Tue, 17 Jan 2017 09:24:26 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Theodore Ts'o <tytso@....edu>
Cc:     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>,
        Jan Kara <jack@...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 16-01-17 21:56:07, Theodore Ts'o wrote:
> On Fri, Jan 06, 2017 at 03:11:07PM +0100, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > This reverts commit 216553c4b7f3e3e2beb4981cddca9b2027523928. Now that
> > the transaction context uses memalloc_nofs_save and all allocations
> > within the this context inherit GFP_NOFS automatically, there is no
> > reason to mark specific allocations explicitly.
> > 
> > This patch should not introduce any functional change. The main point
> > of this change is to reduce explicit GFP_NOFS usage inside ext4 code
> > to make the review of the remaining usage easier.
> > 
> > Signed-off-by: Michal Hocko <mhocko@...e.com>
> > Reviewed-by: Jan Kara <jack@...e.cz>
> 
> Changes in the jbd2 layer aren't going to guarantee that
> memalloc_nofs_save() will be executed if we are running ext4 without a
> journal (aka in no journal mode).  And this is a *very* common
> configuration; it's how ext4 is used inside Google in our production
> servers.

OK, I wasn't aware of that.

> So that means the earlier patches will probably need to be changed so
> the nOFS scope is done in the ext4_journal_{start,stop} functions in
> fs/ext4/ext4_jbd2.c.

I could definitely appreciated some help here. The call paths are rather
complex and I am not familiar with the code enough. On of the biggest
problem I have currently is that there doesn't seem to be an easy place
to store the old allocation context. The original patch had it inside
the journal handle. I was thinking about putting it into superblock but
ext4_journal_stop doesn't seem to have access to the sb if there is no
handle. Now, if ext4_journal_start is never called from a nested context
then this is not a big deal but there are just too many caller to
check...
-- 
Michal Hocko
SUSE Labs
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