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

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Message-ID: <efe00fa8-1a85-f687-b717-e22997dc42d7@redhat.com>
Date:   Sun, 8 May 2022 02:28:51 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Minchan Kim <minchan@...nel.org>
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 05.05.22 08:48, Minchan Kim wrote:
> 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

Sorry for the late reply, still traveling :)

Just so we're on the same page: MIGRATE_ISOLATE would be set on
pageblocks that contain either free or movable pages. In case of movable
pages, they are in uese.

Regarding your is_cma_page() proposal, I think we might want to consider
that if it really turns out to be a problem. For now, I'm fine with just
documenting it.

Thanks!

-- 
Thanks,

David / dhildenb