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

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Date:   Mon, 17 Jul 2023 09:58:39 +0800
From:   Yin Fengwei <fengwei.yin@...el.com>
To:     Yu Zhao <yuzhao@...gle.com>
CC:     <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 7/17/23 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?


Regards
Yin, Fengwei