linux-kernel - Re: [PATCH] mm: move rmap of mTHP upon CoW reuse

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Message-ID: <462bb4e3-7a38-4155-9f95-52fbb4b6d464@redhat.com>
Date: Thu, 25 Sep 2025 12:37:42 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
 rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Lokesh Gidra <lokeshgidra@...gle.com>
Subject: Re: [PATCH] mm: move rmap of mTHP upon CoW reuse

On 25.09.25 12:33, Dev Jain wrote:
> 
> On 25/09/25 2:46 pm, David Hildenbrand wrote:
>> On 25.09.25 10:54, Dev Jain wrote:
>>> At wp-fault time, when we find that a folio is exclusively mapped, we
>>> move
>>> folio->mapping to the faulting VMA's anon_vma, so that rmap overhead
>>> reduces. This is currently done for small folios (base pages) and
>>> PMD-mapped THPs. Do this for mTHP too.
>>
>> I deliberately didn't add this back then because I was not able to
>> convince myself easily that it is ok in all corner cases. So this
>> needs some thought.
> 
> Thanks for your detailed reply.
> 
> 
>>
>>
>> We know that the folio is exclusively mapped to a single MM and that
>> there are no unexpected references from others (GUP pins, whatsoever).
>>
>> But a large folio might be
>>
>> (a) mapped into multiple VMAs (e.g., partial mprotect()) in the same MM
> 
> I think we have the same problem then for PMD-THPs? I see that
> vma_adjust_trans_huge() only does a PMD split and not folio split.

Sure, we can end up in this reuse function here for any large anon 
folio, including PMD ones after a PMD->PTE remapping.

-- 
Cheers

David / dhildenb