Re: [PATCH for v5.9] mm/page_alloc: handle a missing case for memalloc_nocma_{save/restore} APIs

[Joonsoo Kim <js1304@gmail.com>]

2020년 8월 28일 (금) 오전 8:54, Joonsoo Kim <js1304@gmail.com>님이 작성:
>
> 2020년 8월 27일 (목) 오후 10:35, Mel Gorman <mgorman@techsingularity.net>님이 작성:
> >
> > On Wed, Aug 26, 2020 at 02:12:44PM +0900, Joonsoo Kim wrote:
> > > > > And, it requires to break current code
> > > > > layering that order-0 page is always handled by the pcplist. I'd prefer
> > > > > to avoid it so this patch uses different way to skip CMA page allocation
> > > > > from the pcplist.
> > > >
> > > > Well it would be much simpler and won't affect most of allocations. Better than
> > > > flushing pcplists IMHO.
> > >
> > > Hmm...Still, I'd prefer my approach.
> >
> > I prefer the pcp bypass approach. It's simpler and it does not incur a
> > pcp drain/refill penalty.
> >
> > > There are two reasons. First,
> > > layering problem
> > > mentioned above. In rmqueue(), there is a code for MIGRATE_HIGHATOMIC.
> > > As the name shows, it's for high order atomic allocation. But, after
> > > skipping pcplist
> > > allocation as you suggested, we could get there with order 0 request.
> >
> > I guess your concern is that under some circumstances that a request that
> > passes a watermark check could fail due to a highatomic reserve and to
> > an extent this is true. However, in that case the system is already low
> > on memory depending on the allocation context, the pcp lists may get
> > flushed anyway.
>
> My concern is that non-highorder (order-0) allocation could pollute/use the
> MIGRATE_HIGHATOMIC pageblock. It's reserved for highorder atomic
> allocation so it's not good if an order-0 request could get there. It would
> cause more fragmentation on that pageblock.
>
> > > We can also
> > > change this code, but, I'd hope to maintain current layering. Second,
> > > a performance
> > > reason. After the flag for nocma is up, a burst of nocma allocation
> > > could come. After
> > > flushing the pcplist one times, we can use the free page on the
> > > pcplist as usual until
> > > the context is changed.
> >
> > It's not guaranteed because CMA pages could be freed between the nocma save
> > and restore triggering further drains due to a reschedule.  Similarly,
> > a CMA allocation in parallel could refill with CMA pages on the per-cpu
> > list. While both cases are unlikely, it's more unpredictable than a
> > straight-forward pcp bypass.
>
> Agreed that it's unpredictable than the pcp bypass. But, as you said,
> those cases
> would be rare.
>
> > I don't really see it as a layering violation of the API because all
> > order-0 pages go through the PCP lists. The fact that order-0 is serviced
> > from the pcp list is an internal implementation detail, the API doesn't
> > care.
>
> What I mean is an internal implementation layering violation. We could make
> a rule even in internal implementation to make code simpler and maintainable.
> I guess that order-0 is serviced from the pcp list is one of those.
>
> Anyway, although I prefer my approach, I'm okay with using pcp bypass.

Hello, Andrew and Vlastimil.

It's better to fix this possible bug introduced in v5.9-rc1 before
v5.9 is released.
Which approach do you prefer?
If it is determined, I will immediately send a patch as you suggested.

Thanks.