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

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Message-ID: <20180529081823.GN27180@dhcp22.suse.cz>
Date:   Tue, 29 May 2018 10:18:23 +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 Tue 29-05-18 08:32:05, Dave Chinner wrote:
> On Mon, May 28, 2018 at 11:19:23AM +0200, Michal Hocko wrote:
> > 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
> 
> .... restore function should be called when ...

fixed

> But otherwise I think this is much better.

Thanks!

-- 
Michal Hocko
SUSE Labs