linux-kernel - Re: [PATCH] mm: fix is_pinnable

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Date:   Wed, 4 May 2022 23:48:30 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        John Dias <joaodias@...gle.com>
Subject: Re: [PATCH] mm: fix is_pinnable_page against on cma page

On Wed, May 04, 2022 at 03:48:54PM -0700, Minchan Kim wrote:
> On Tue, May 03, 2022 at 06:02:33PM +0200, David Hildenbrand wrote:
> > On 03.05.22 17:26, Minchan Kim wrote:
> > > On Tue, May 03, 2022 at 03:15:24AM +0200, David Hildenbrand wrote:
> > >>
> > >>>>>> However, I assume we have the same issue right now already with
> > >>>> ZONE_MOVABLE and MIGRATE_CMA when trying to pin a page residing on these
> > >>>
> > >>> ZONE_MOVALBE is also changed dynamically?
> > >>>
> > >>
> > >> Sorry, with "same issue" I meant failing to pin if having to migrate and
> > >> the page is temporarily unmovable.
> > >>
> > >>>> there are temporarily unmovable and we fail to migrate. But it would now
> > >>>> apply even without ZONE_MOVABLE or MIGRATE_CMA. Hm...
> > >>>
> > >>> Didn't parse your last mention.
> > >>
> > >> On a system that neither uses ZONE_MOVABLE nor MIGRATE_CMA we might have
> > >> to migrate now when pinning.
> > > 
> > > I don't understand your point. My problem is pin_user_pages with
> > > FOLL_LONGTERM. It shouldn't pin a page from ZONE_MOVABLE and cma area
> > > without migrating page out of movable zone or CMA area.
> > > That's why try_grab_folio checks whether target page stays in those
> > > movable areas. However, to check CMA area, is_migrate_cma_page is
> > > racy so the FOLL_LONGTERM flag semantic is broken right now.
> > > 
> > > Do you see any problem of the fix?
> > 
> > My point is that you might decide to migrate a page because you stumble
> > over MIGRATE_ISOLATE, although there is no need to reject long-term
> > pinning and to trigger page migration.
> > 
> > Assume a system without ZONE_MOVABLE and without MIGRATE_CMA. Assume
> > someone reserves gigantic pages (alloc_contig_range()) and you have
> > concurrent long-term pinning on a page that is no MIGRATE_ISOLATE.
> > 
> > GUP would see MIGRATE_ISOLATE and would reject pinning. The page has to
> > be migrated, which can fail if the page is temporarily unmovable.
> 
> A dump question since I'm not familiar with hugetlb.
> 
> Is above reasonable scenario?
> 
> The gigantic page is about to be created using alloc_contig_range so
> they has MIGRATE_ISOLATE as temporal state. It means no one uses the
> page yet so I guess the page is not mapped at userspace but other is
> trying to access the page using pin_user_pages?
> 

Too dump question. Never mind.
Posted v2 - https://lore.kernel.org/all/20220505064429.2818496-1-minchan@kernel.org/T/#u

Thanks.