Re: [PATCH] mm: fix is_pinnable_page against on cma page

From: Minchan Kim <minchan@kernel.org>
To: David Hildenbrand <david@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	John Hubbard <jhubbard@nvidia.com>,
	John Dias <joaodias@google.com>
Subject: Re: [PATCH] mm: fix is_pinnable_page against on cma page
Date: Tue, 3 May 2022 08:26:42 -0700	[thread overview]
Message-ID: <YnFJss0doXGCmq3w@google.com> (raw)
In-Reply-To: <08e9855c-395d-f40c-de3d-1ec8b644bfe8@redhat.com>

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?

A thing to get some attention is whether we need READ_ONCE or not
for the local variable mt.