linux-kernel - Re: [PATCH] doc: document scope NOFS, NOIO APIs

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Message-ID: <20180525081624.GH11881@dhcp22.suse.cz>
Date:   Fri, 25 May 2018 10:16:24 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Jonathan Corbet <corbet@....net>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        David Sterba <dsterba@...e.cz>
Subject: Re: [PATCH] doc: document scope NOFS, NOIO APIs

On Fri 25-05-18 08:17:15, Dave Chinner wrote:
> On Thu, May 24, 2018 at 01:43:41PM +0200, Michal Hocko wrote:
[...]
> > +FS/IO code then simply calls the appropriate save function right at the
> > +layer where a lock taken from the reclaim context (e.g. shrinker) and
> > +the corresponding restore function when the lock is released. All that
> > +ideally along with an explanation what is the reclaim context for easier
> > +maintenance.
> 
> This paragraph doesn't make much sense to me. I think you're trying
> to say that we should call the appropriate save function "before
> locks are taken that a reclaim context (e.g a shrinker) might
> require access to."
> 
> I think it's also worth making a note about recursive/nested
> save/restore stacking, because it's not clear from this description
> that this is allowed and will work as long as inner save/restore
> calls are fully nested inside outer save/restore contexts.

Any better?

-FS/IO code then simply calls the appropriate save function right at the
-layer where a lock taken from the reclaim context (e.g. shrinker) and
-the corresponding restore function when the lock is released. All that
-ideally along with an explanation what is the reclaim context for easier
-maintenance.
+FS/IO code then simply calls the appropriate save function before any
+lock shared with the reclaim context is taken.  The corresponding
+restore function when the lock is released. All that ideally along with
+an explanation what is the reclaim context for easier maintenance.
+
+Please note that the proper pairing of save/restore function allows nesting
+so memalloc_noio_save is safe to be called from an existing NOIO or NOFS scope.
 
 What about __vmalloc(GFP_NOFS)
 ==============================
-- 
Michal Hocko
SUSE Labs