linux-kernel - Re: [PATCH 4/6] mm cma: rename PF_MEMALLOC_NOCMA to PF_MEMALLOC

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Message-ID: <CA+CK2bAqXcMjxo9F5_0HP8kPbDjnewxcTpVNBbaVZZjhM4FMEg@mail.gmail.com>
Date:   Thu, 3 Dec 2020 10:02:01 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 4/6] mm cma: rename PF_MEMALLOC_NOCMA to PF_MEMALLOC_NOMOVABLE

On Thu, Dec 3, 2020 at 3:57 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Wed 02-12-20 00:23:28, Pavel Tatashin wrote:
> > PF_MEMALLOC_NOCMA is used for longterm pinning and has an effect of
> > clearing _GFP_MOVABLE or prohibiting allocations from ZONE_MOVABLE.
>
> This is not precise. You are mixing the implementation and the intention
> here. I would say "PF_MEMALLOC_NOCMA is used ot guarantee that the
> allocator will not return pages that might belong to CMA region. This is
> currently used for long term gup to make sure that such pins are not
> going to be done on any CMA pages."
>
> > We will prohibit allocating any pages that are getting
> > longterm pinned from ZONE_MOVABLE, and we would want to unify and re-use
> > this flag. So, rename it to generic PF_MEMALLOC_NOMOVABLE.
> > Also re-name:
> > memalloc_nocma_save()/memalloc_nocma_restore
> > to
> > memalloc_nomovable_save()/memalloc_nomovable_restore()
> > and make the new functions common.
>
> This is hard to parse for me. I would go with something like:
> "
> When PF_MEMALLOC_NOCMA has been introduced we haven't realized that it
> is focusing on CMA pages too much and that there is larger class of
> pages that need the same treatment. MOVABLE zone cannot contain
> any long term pins as well so it makes sense to reuse and redefine this
> flag for that usecase as well. Rename the flag to PF_MEMALLOC_NOMOVABLE
> which defines an allocation context which can only get pages suitable
> for long-term pins.
> "

I will address the above with your suggested wording.

>
> Btw. the naming is hard but PF_MEMALLOC_NOMOVABLE is a bit misnomer. CMA
> pages are not implicitly movable. So in fact we do care more about
> pinning than movability. PF_MEMALLOC_PIN or something similar would be
> better fit for the overal intention.

Sounds good, I will rename with PF_MEMALLOC_PIN

>
> Other than that looks good to me. Thanks!

Thank you,
Pasha