linux-kernel - Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte

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Date:   Wed, 19 Jul 2023 07:47:38 +0800
From:   Yin Fengwei <fengwei.yin@...el.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
CC:     Yu Zhao <yuzhao@...gle.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
        <willy@...radead.org>, <david@...hat.com>, <ryan.roberts@....com>,
        <shy828301@...il.com>
Subject: Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte_range to handle
 large folio



On 7/19/23 06:48, Yosry Ahmed wrote:
> On Sun, Jul 16, 2023 at 6:58 PM Yin Fengwei <fengwei.yin@...el.com> wrote:
>>
>>
>>
>> On 7/17/23 08:35, Yu Zhao wrote:
>>> On Sun, Jul 16, 2023 at 6:00 PM Yin, Fengwei <fengwei.yin@...el.com> wrote:
>>>>
>>>> On 7/15/2023 2:06 PM, Yu Zhao wrote:
>>>>> There is a problem here that I didn't have the time to elaborate: we
>>>>> can't mlock() a folio that is within the range but not fully mapped
>>>>> because this folio can be on the deferred split queue. When the split
>>>>> happens, those unmapped folios (not mapped by this vma but are mapped
>>>>> into other vmas) will be stranded on the unevictable lru.
>>>>
>>>> This should be fine unless I missed something. During large folio split,
>>>> the unmap_folio() will be migrate(anon)/unmap(file) folio. Folio will be
>>>> munlocked in unmap_folio(). So the head/tail pages will be evictable always.
>>>
>>> It's close but not entirely accurate: munlock can fail on isolated folios.
>> Yes. The munlock just clear PG_mlocked bit but with PG_unevictable left.
>>
>> Could this also happen against normal 4K page? I mean when user try to munlock
>> a normal 4K page and this 4K page is isolated. So it become unevictable page?
> 
> Looks like it can be possible. If cpu 1 is in __munlock_folio() and
> cpu 2 is isolating the folio for any purpose:
> 
> cpu1                                        cpu2
>                                                 isolate folio
> folio_test_clear_lru() // 0
>                                                 putback folio // add
> to unevictable list
> folio_test_clear_mlocked()
Yes. Yu showed this sequence to me in another email. I thought the putback_lru()
could correct the none-mlocked but unevictable folio. But it doesn't because
of this race.

> 
> 
> The page would be stranded on the unevictable list in this case, no?
> Maybe we should only try to isolate the page (clear PG_lru) after we
> possibly clear PG_mlocked? In this case if we fail to isolate we know
> for sure that whoever has the page isolated will observe that
> PG_mlocked is clear and correctly make the page evictable.
> 
> This probably would be complicated with the current implementation, as
> we first need to decrement mlock_count to determine if we want to
> clear PG_mlocked, and to do so we need to isolate the page as
> mlock_count overlays page->lru. With the proposal in [1] to rework
> mlock_count, it might be much simpler as far as I can tell. I intend
> to refresh this proposal soon-ish.
> 
> [1]https://lore.kernel.org/lkml/20230618065719.1363271-1-yosryahmed@google.com/
> 
>>
>>
>> Regards
>> Yin, Fengwei
>>