linux-kernel - Re: Memory allocation on speculative fastpaths

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Date:   Wed, 25 May 2022 09:12:05 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        "Liam R. Howlett" <liam.howlett@...cle.com>,
        Michel Lespinasse <walken.cr@...il.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: Memory allocation on speculative fastpaths

On Tue, May 24, 2022 at 10:37:15PM +0200, Vlastimil Babka wrote:
> On 5/4/22 18:23, Johannes Weiner wrote:
> > On Tue, May 03, 2022 at 04:15:46PM -0700, Suren Baghdasaryan wrote:
> >> On Tue, May 3, 2022 at 11:28 AM Matthew Wilcox <willy@...radead.org> wrote:
> >>>
> >>> On Tue, May 03, 2022 at 09:39:05AM -0700, Paul E. McKenney wrote:
> >>>> On Tue, May 03, 2022 at 06:04:13PM +0200, Michal Hocko wrote:
> >>>>> On Tue 03-05-22 08:59:13, Paul E. McKenney wrote:
> >>>>>> Hello!
> >>>>>>
> >>>>>> Just following up from off-list discussions yesterday.
> >>>>>>
> >>>>>> The requirements to allocate on an RCU-protected speculative fastpath
> >>>>>> seem to be as follows:
> >>>>>>
> >>>>>> 1.        Never sleep.
> >>>>>> 2.        Never reclaim.
> >>>>>> 3.        Leave emergency pools alone.
> >>>>>>
> >>>>>> Any others?
> >>>>>>
> >>>>>> If those rules suffice, and if my understanding of the GFP flags is
> >>>>>> correct (ha!!!), then the following GFP flags should cover this:
> >>>>>>
> >>>>>>   __GFP_NOMEMALLOC | __GFP_NOWARN
> >>>>>
> >>>>> GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN
> >>>>
> >>>> Ah, good point on GFP_NOWAIT, thank you!
> >>>
> >>> Johannes (I think it was?) made the point to me that if we have another
> >>> task very slowly freeing memory, a task in this path can take advantage
> >>> of that other task's hard work and never go into reclaim.  So the
> >>> approach we should take is:
> > 
> > Right, GFP_NOWAIT can starve out other allocations. It can clear out
> > the freelists without the burden of having to do reclaim like
> > everybody else wanting memory during a shortage. Including GFP_KERNEL.
> 
> FTR, I wonder if this is really true, given the suggested fallback.

                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

IIRC adding this fallback was the conclusion of the in-person
discussion. Above I just tried to summarize for the record the
original concern that led to it. I could have been more clear.

Your analysis is dead on, of course.