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 26일 (수) 오전 9:42, Andrew Morton <akpm@linux-foundation.org>님이 작성:
>
> 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 feature
> > > 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 the
> > > page back if it's CMA and go get another one?
> >
> > Hmm... Unfortunately, it cannot ensure that we eventually get the non-CMA 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.