linux-kernel - Re: [PATCH 2/4] mm/gup: restrict CMA region by using allocation scope API

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Date:   Fri, 17 Jul 2020 10:26:43 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@....com,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Hellwig <hch@...radead.org>,
        Roman Gushchin <guro@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH 2/4] mm/gup: restrict CMA region by using allocation
 scope API

On Fri 17-07-20 16:46:38, Joonsoo Kim wrote:
> 2020년 7월 15일 (수) 오후 5:24, Michal Hocko <mhocko@...nel.org>님이 작성:
> >
> > On Wed 15-07-20 14:05:27, Joonsoo Kim wrote:
> > > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > >
> > > We have well defined scope API to exclude CMA region.
> > > Use it rather than manipulating gfp_mask manually. With this change,
> > > we can now use __GFP_MOVABLE for gfp_mask and the ZONE_MOVABLE is also
> > > searched by page allocator. For hugetlb, gfp_mask is redefined since
> > > it has a regular allocation mask filter for migration target.
> > >
> > > Note that this can be considered as a fix for the commit 9a4e9f3b2d73
> > > ("mm: update get_user_pages_longterm to migrate pages allocated from
> > > CMA region"). However, "Fixes" tag isn't added here since it is just
> > > suboptimal but it doesn't cause any problem.
> >
> > But it is breaking the contract that the longterm pins never end up in a
> > cma managed memory. So I think Fixes tag is really due. I am not sure
> > about stable backport. If the patch was the trivial move of
> 
> Previous implementation is correct since longterm pins never end up in a CMA
> managed memory with that implementation. It's just a different and suboptimal
> implementation to exclude the CMA area. This is why I don't add the "Fixes"A
> tag on the patch.

But the current implementation calls memalloc_nocma_restore too early so
__gu_longterm_locked will migrate pages possibly to CMA ranges as there
is no GFP_MOVABLE restriction in place. Or am I missing something?

-- 
Michal Hocko
SUSE Labs