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From:   Joonsoo Kim <js1304@gmail.com>
Date:   Wed, 26 Aug 2020 14:12:44 +0900
Message-ID: <CAAmzW4MRizKRbdt_ZBqs_+OQGsP4f2F6iYDk88u78kYqOZA9NQ@mail.gmail.com>
Subject: Re: [PATCH for v5.9] mm/page_alloc: handle a missing case for
 memalloc_nocma_{save/restore} APIs
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
        kernel-team@lge.com, Joonsoo Kim <iamjoonsoo.kim@lge.com>
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2020=EB=85=84 8=EC=9B=94 25=EC=9D=BC (=ED=99=94) =EC=98=A4=ED=9B=84 6:43, V=
lastimil Babka <vbabka@suse.cz>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:
>
>
> On 8/25/20 6:59 AM, js1304@gmail.com wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >
> > memalloc_nocma_{save/restore} APIs can be used to skip page allocation
> > on CMA area, but, there is a missing case and the page on CMA area coul=
d
> > be allocated even if APIs are used. This patch handles this case to fix
> > the potential issue.
> >
> > Missing case is an allocation from the pcplist. MIGRATE_MOVABLE pcplist
> > could have the pages on CMA area so we need to skip it if ALLOC_CMA isn=
't
> > specified.
> >
> > This patch implements this behaviour by checking allocated page from
> > the pcplist rather than skipping an allocation from the pcplist entirel=
y.
> > Skipping the pcplist entirely would result in a mismatch between waterm=
ark
> > check and actual page allocation.
>
> Are you sure? I think a mismatch exists already. Pages can be on the pcpl=
ist but
> they are not considered as free in the watermark check. So passing waterm=
ark
> check means there should be also pages on free lists. So skipping pcplist=
s would
> be safe, no?

You are right.

> > 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 allocatio=
n
> > from the pcplist.
>
> Well it would be much simpler and won't affect most of allocations. Bette=
r than
> flushing pcplists IMHO.

Hmm...Still, I'd prefer my approach. 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.
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.

How about my reasoning?

Thanks.