linux-kernel - Re: [PATCH v4 04/11] mm/hugetlb: make hugetlb migration callback CMA aware

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Date:   Thu, 9 Jul 2020 16:03:27 +0900
From:   Joonsoo Kim <js1304@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@....com,
        Christoph Hellwig <hch@...radead.org>,
        Roman Gushchin <guro@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH v4 04/11] mm/hugetlb: make hugetlb migration callback CMA aware

2020년 7월 9일 (목) 오후 3:43, Michal Hocko <mhocko@...nel.org>님이 작성:
>
> On Wed 08-07-20 09:41:06, Michal Hocko wrote:
> > On Wed 08-07-20 16:16:02, Joonsoo Kim wrote:
> > > On Tue, Jul 07, 2020 at 01:22:31PM +0200, Vlastimil Babka wrote:
> > > > On 7/7/20 9:44 AM, js1304@...il.com wrote:
> > > > > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > > > >
> > > > > new_non_cma_page() in gup.c which try to allocate migration target page
> > > > > requires to allocate the new page that is not on the CMA area.
> > > > > new_non_cma_page() implements it by removing __GFP_MOVABLE flag.  This way
> > > > > works well for THP page or normal page but not for hugetlb page.
> > > > >
> > > > > hugetlb page allocation process consists of two steps.  First is dequeing
> > > > > from the pool.  Second is, if there is no available page on the queue,
> > > > > allocating from the page allocator.
> > > > >
> > > > > new_non_cma_page() can control allocation from the page allocator by
> > > > > specifying correct gfp flag.  However, dequeing cannot be controlled until
> > > > > now, so, new_non_cma_page() skips dequeing completely.  It is a suboptimal
> > > > > since new_non_cma_page() cannot utilize hugetlb pages on the queue so this
> > > > > patch tries to fix this situation.
> > > > >
> > > > > This patch makes the deque function on hugetlb CMA aware and skip CMA
> > > > > pages if newly added skip_cma argument is passed as true.
> > > >
> > > > Hmm, can't you instead change dequeue_huge_page_node_exact() to test the PF_
> > > > flag and avoid adding bool skip_cma everywhere?
> > >
> > > Okay! Please check following patch.
> > > >
> > > > I think that's what Michal suggested [1] except he said "the code already does
> > > > by memalloc_nocma_{save,restore} API". It needs extending a bit though, AFAICS.
> > > > __gup_longterm_locked() indeed does the save/restore, but restore comes before
> > > > check_and_migrate_cma_pages() and thus new_non_cma_page() is called, so an
> > > > adjustment is needed there, but that's all?
> > > >
> > > > Hm the adjustment should be also done because save/restore is done around
> > > > __get_user_pages_locked(), but check_and_migrate_cma_pages() also calls
> > > > __get_user_pages_locked(), and that call not being between nocma save and
> > > > restore is thus also a correctness issue?
> > >
> > > Simply, I call memalloc_nocma_{save,restore} in new_non_cma_page(). It
> > > would not cause any problem.
> >
> > I believe a proper fix is the following. The scope is really defined for
> > FOLL_LONGTERM pins and pushing it inside check_and_migrate_cma_pages
> > will solve the problem as well but it imho makes more sense to do it in
> > the caller the same way we do for any others.
> >
> > Fixes: 9a4e9f3b2d73 ("mm: update get_user_pages_longterm to migrate pages allocated from CMA region")
> >
> > I am not sure this is worth backporting to stable yet.
>
> Should I post it as a separate patch do you plan to include this into your next version?

It's better to include it on my next version since this patch would
cause a conflict with
the next tree that includes my v3 of this patchset.

Thanks.