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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Wed, 1 Apr 2020 10:45:56 -0400
Message-ID: <CAEXW_YTpXojYiskwiqZGHpT45v3xZYhuvy0CubaeyB3fMrmw7g@mail.gmail.com>
Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free
 memory pattern
To:     Mel Gorman <mgorman@suse.de>
Cc:     Michal Hocko <mhocko@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, rcu <rcu@vger.kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Neil Brown <neilb@suse.com>, Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
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On Wed, Apr 1, 2020 at 9:14 AM Mel Gorman <mgorman@suse.de> wrote:
>
> On Wed, Apr 01, 2020 at 09:23:59AM +0200, Michal Hocko wrote:
> > > Can you suggest what prevents other users of GFP_MEMALLOC from doing that
> > > also?
> >
> > There is no explicit mechanism which is indeed unfortunate. The only
> > user real user of the flag is Swap over NFS AFAIK. I have never dared to
> > look into details on how the complete reserves depletion is prevented.
> > Mel would be much better fit here.
> >
>
> It's "prevented" by the fact that every other memory allocation request
> that is not involved with reclaiming memory gets stalled in the allocator
> with only the swap subsystem making any progress until the machine
> recovers. Potentially only kswapd is still running until the system
> recovers if stressed hard enough.
>
> The naming is terrible but is mased on kswapd's use of the PF_MEMALLOC
> flag. For swap-over-nfs, GFP_MEMALLOC saying "this allocation request is
> potentially needed for kswapd to make forward progress and not freeze".
>
> I would not be comfortable with kfree_rcu() doing the same thing because
> there can be many callers in parallel and it's freeing slab objects.
> Swap over NFS should free at least one page, freeing a slab object is
> not guaranteed to free anything.

Got it Mel. Just to clarify to the onlooker. It seemed to fit the
pattern that's why I proposed it as RFC, I was never sure it was the
right approach -- I just proposed it for discussion-sake because I
thought it was worth talking about at least. It was not even merged in
my tree, was just RFC.

Thanks Mel for clarifying the usage of the flag.

 - Joel