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

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Date:   Wed, 8 Jul 2020 13:32:54 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, Joonsoo Kim <js1304@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        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>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4 04/11] mm/hugetlb: make hugetlb migration callback CMA
 aware

On Wed 08-07-20 16:27:16, Aneesh Kumar K.V wrote:
> Vlastimil Babka <vbabka@...e.cz> writes:
> 
> > On 7/8/20 9:41 AM, 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:
> >>> 
> >>> 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")
> >
> > Agreed.
> >
> >> 
> >> I am not sure this is worth backporting to stable yet.
> >
> > CC Aneesh.
> >
> > Context: since check_and_migrate_cma_pages() calls __get_user_pages_locked(), it
> > should also be called under memalloc_nocma_save().
> 
> But by then we faulted in all relevant pages and migrated them out of
> CMA rea right?

check_and_migrate_cma_pages will allocate target pages that you want to
migrate off the CMA region unless I am misreading the code. And those
allocation need to be placed outside of the CMA.
-- 
Michal Hocko
SUSE Labs