linux-kernel - Re: [PATCH 4/7] mm: introduce memalloc_nofs

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Date:   Thu, 9 Mar 2017 12:42:53 +0100
From:   David Sterba <dsterba@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Dave Chinner <david@...morbit.com>, djwong@...nel.org,
        Theodore Ts'o <tytso@....edu>, 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>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 4/7] mm: introduce memalloc_nofs_{save,restore} API

On Tue, Mar 07, 2017 at 04:09:56PM +0100, Michal Hocko wrote:
> On Mon 06-03-17 13:22:14, Andrew Morton wrote:
> > On Mon,  6 Mar 2017 14:14:05 +0100 Michal Hocko <mhocko@...nel.org> wrote:
> [...]
> > > --- a/include/linux/gfp.h
> > > +++ b/include/linux/gfp.h
> > > @@ -210,8 +210,16 @@ struct vm_area_struct;
> > >   *
> > >   * GFP_NOIO will use direct reclaim to discard clean pages or slab pages
> > >   *   that do not require the starting of any physical IO.
> > > + *   Please try to avoid using this flag directly and instead use
> > > + *   memalloc_noio_{save,restore} to mark the whole scope which cannot
> > > + *   perform any IO with a short explanation why. All allocation requests
> > > + *   will inherit GFP_NOIO implicitly.
> > >   *
> > >   * GFP_NOFS will use direct reclaim but will not use any filesystem interfaces.
> > > + *   Please try to avoid using this flag directly and instead use
> > > + *   memalloc_nofs_{save,restore} to mark the whole scope which cannot/shouldn't
> > > + *   recurse into the FS layer with a short explanation why. All allocation
> > > + *   requests will inherit GFP_NOFS implicitly.
> > 
> > I wonder if these are worth a checkpatch rule.
> 
> I am not really sure, to be honest. This may easilly end up people
> replacing
> 
> do_alloc(GFP_NOFS)
> 
> with
> 
> memalloc_nofs_save()
> do_alloc(GFP_KERNEL)
> memalloc_nofs_restore()
> 
> which doesn't make any sense of course. From my experience, people tend
> to do stupid things just to silent checkpatch warnings very often.
> Moreover I believe we need to do the transition to the new api first
> before we can push back on the explicit GFP_NOFS usage. Maybe then we
> can think about the a checkpatch warning.

I agree will all your objections against adding that to checkpatch, at
this point it's less harmful to use GFP_NOFS.