linux-kernel - Re: [PATCH v5 5/5] mm: rmap: support batched unmapping for file large folios

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Message-ID: <467ab4d7-1ce2-48d4-bff6-6579216569e2@arm.com>
Date: Sun, 18 Jan 2026 11:18:16 +0530
From: Dev Jain <dev.jain@....com>
To: Barry Song <21cnbao@...il.com>
Cc: Wei Yang <richard.weiyang@...il.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 david@...nel.org, catalin.marinas@....com, will@...nel.org,
 lorenzo.stoakes@...cle.com, ryan.roberts@....com, Liam.Howlett@...cle.com,
 vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
 riel@...riel.com, harry.yoo@...cle.com, jannh@...gle.com,
 willy@...radead.org, linux-mm@...ck.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 5/5] mm: rmap: support batched unmapping for file large
 folios


On 16/01/26 8:44 pm, Barry Song wrote:
>>> I mean maybe we can skip it in try_to_unmap_one(), for example:
>>>
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 9e5bd4834481..ea1afec7c802 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -2250,6 +2250,10 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>>                */
>>>               if (nr_pages == folio_nr_pages(folio))
>>>                       goto walk_done;
>>> +             else {
>>> +                     pvmw.address += PAGE_SIZE * (nr_pages - 1);
>>> +                     pvmw.pte += nr_pages - 1;
>>> +             }
>>>               continue;
>>>  walk_abort:
>>>               ret = false;
>> I am of the opinion that we should do something like this. In the internal pvmw code,
>> we keep skipping ptes till the ptes are none. With my proposed uffd-fix [1], if the old
>> ptes were uffd-wp armed, pte_install_uffd_wp_if_needed will convert all ptes from none
>> to not none, and we will lose the batching effect. I also plan to extend support to
>> anonymous folios (therefore generalizing for all types of memory) which will set a
> I posted an RFC on anon folios quite some time ago [1].
> It’s great to hear that you’re interested in taking this over.
>
> [1] https://lore.kernel.org/all/20250513084620.58231-1-21cnbao@gmail.com/

Great! Now I have a reference to look at :)

>
>> batch of ptes as swap, and the internal pvmw code won't be able to skip through the
>> batch.
> Interesting — I didn’t catch this issue in the RFC earlier. Back then,
> we only supported nr == 1 and nr == folio_nr_pages(folio). When
> nr == nr_pages, page_vma_mapped_walk() would break entirely. With
> Lance’s commit ddd05742b45b08, arbitrary nr in [1, nr_pages] is now
> supported, which means we have to handle all the complexity. :-)
>
> Thanks
> Barry