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

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Message-Id: <20251023134929.b72ab75a8c132a17ae68a582@linux-foundation.org>
Date: Thu, 23 Oct 2025 13:49:29 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Kiryl Shutsemau <kirill@...temov.name>
Cc: David Hildenbrand <david@...hat.com>, 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 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...