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

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Date:   Mon, 28 May 2018 11:19:23 +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 Mon 28-05-18 09:48:54, Dave Chinner wrote:
> On Fri, May 25, 2018 at 10:16:24AM +0200, Michal Hocko wrote:
> > 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.
> 
> It's better, but the talk of this being necessary for locking makes
> me cringe. XFS doesn't do it for locking reasons - it does it
> largely for preventing transaction context nesting, which has all
> sorts of problems that cause hangs (e.g. log space reservations
> can't be filled) that aren't directly locking related.

Yeah, I wanted to not mention locks as much as possible.
 
> i.e we should be talking about using these functions around contexts
> where recursion back into the filesystem through reclaim is
> problematic, not that "holding locks" is problematic. Locks can be
> used as an example of a problematic context, but locks are not the
> only recursion issue that require GFP_NOFS allocation contexts to
> avoid.

agreed. Do you have any suggestion how to add a more abstract wording
that would not make head spinning?

I've tried the following. Any better?

diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst
index c0ec212d6773..adac362b2875 100644
--- a/Documentation/core-api/gfp_mask-from-fs-io.rst
+++ b/Documentation/core-api/gfp_mask-from-fs-io.rst
@@ -34,9 +34,11 @@ scope will inherently drop __GFP_FS respectively __GFP_IO from the given
 mask so no memory allocation can recurse back in the FS/IO.
 
 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.
+critical section wrt. the reclaim is started - e.g. lock shared with the
+reclaim context or when a transaction context nesting would be possible
+via reclaim. The corresponding restore function when the critical
+section ends. 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.
-- 
Michal Hocko
SUSE Labs