linux-kernel - Re: [PATCHv2 1/2] mm/memory: Do not populate page table entries beyond i

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Message-ID: <3ad31422-81c7-47f2-ae3e-e6bc1df402ee@redhat.com>
Date: Thu, 23 Oct 2025 22:54:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
 Kiryl Shutsemau <kirill@...temov.name>
Cc: Hugh Dickins <hughd@...gle.com>, Matthew Wilcox <willy@...radead.org>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Rik van Riel <riel@...riel.com>, Harry Yoo <harry.yoo@...cle.com>,
 Johannes Weiner <hannes@...xchg.org>, Shakeel Butt <shakeel.butt@...ux.dev>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Darrick J. Wong" <djwong@...nel.org>, Dave Chinner <david@...morbit.com>,
 linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, Kiryl Shutsemau <kas@...nel.org>
Subject: Re: [PATCHv2 1/2] mm/memory: Do not populate page table entries
 beyond i_size

On 23.10.25 22:49, Andrew Morton wrote:
> On Thu, 23 Oct 2025 10:32:50 +0100 Kiryl Shutsemau <kirill@...temov.name> wrote:
> 
>> From: Kiryl Shutsemau <kas@...nel.org>
>>
>> Accesses within VMA, but beyond i_size rounded up to PAGE_SIZE are
>> supposed to generate SIGBUS.
>>
>> Recent changes attempted to fault in full folio where possible. They did
>> not respect i_size, which led to populating PTEs beyond i_size and
>> breaking SIGBUS semantics.
>>
>> Darrick reported generic/749 breakage because of this.
>>
>> However, the problem existed before the recent changes. With huge=always
>> tmpfs, any write to a file leads to PMD-size allocation. Following the
>> fault-in of the folio will install PMD mapping regardless of i_size.
>>
>> Fix filemap_map_pages() and finish_fault() to not install:
>>    - PTEs beyond i_size;
>>    - PMD mappings across i_size;
>>
>> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
>> Fixes: 19773df031bc ("mm/fault: try to map the entire file folio in finish_fault()")
> 
> Sep 28 2025
> 
>> Fixes: 357b92761d94 ("mm/filemap: map entire large folio faultaround")
> 
> Sep 28 2025
> 
>> Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
> 
> Jul 26 2016
> 
> eek, what's this one doing here?  Are we asking -stable maintainers
> to backport this patch into nine years worth of kernels?
> 
> I'll remove this Fixes: line for now...

Ehm, why? It sure is a fix for that. We can indicate to which stable 
versions we want to have ti backported.

And yes, it might be all stable kernels.

-- 
Cheers

David / dhildenb