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From:   Joonsoo Kim <js1304@gmail.com>
Date:   Wed, 26 Aug 2020 14:21:44 +0900
Message-ID: <CAAmzW4PLv_6wfJFsDKGNziHwAS94ERJL4tEwPLuUCpCDC1DnAA@mail.gmail.com>
Subject: Re: [PATCH for v5.9] mm/page_alloc: handle a missing case for
 memalloc_nocma_{save/restore} APIs
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        "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 26=EC=9D=BC (=EC=88=98) =EC=98=A4=EC=A0=84 9:42, A=
ndrew Morton <akpm@linux-foundation.org>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=
=B1:
>
> On Tue, 25 Aug 2020 14:34:32 +0900 Joonsoo Kim <js1304@gmail.com> wrote:
>
> > >
> > > That's a bunch more code on a very hot path to serve an obscure featu=
re
> > > which has a single obscure callsite.
> > >
> > > Can we instead put the burden on that callsite rather than upon
> > > everyone?  For (dumb) example, teach __gup_longterm_locked() to put t=
he
> > > page back if it's CMA and go get another one?
> >
> > Hmm... Unfortunately, it cannot ensure that we eventually get the non-C=
MA page.
> > I think that the only way to ensure it is to implement the
> > functionality here. We can
> > use 'unlikely' or 'static branch' to reduce the overhead for a really
> > rare case but
> > for now I have no idea how to completely remove the overhead.
>
> Gee, there must be something?  Provide the gup code with a special
> entry point which takes the page straight from __rmqueue() and bypasses
> the pcp lists?

Hmm... it seems not possible. It's allocation context API and maybe actual
allocation happens in handle_mm_fault() or it's successor. We cannot use
a special entry point for allocation there since it's a general function.

And, IMHO, making a special allocation function that bypasses the pcp list
would not be a good practice.

Thanks.