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

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Message-ID: <18262e42-9686-43c1-8f5f-0595b5a00de1@redhat.com>
Date: Fri, 24 Oct 2025 17:42:38 +0200
From: David Hildenbrand <david@...hat.com>
To: Kiryl Shutsemau <kirill@...temov.name>,
 Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>,
 Matthew Wilcox <willy@...radead.org>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>
Cc: 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 11:32, Kiryl Shutsemau 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()")
> Fixes: 357b92761d94 ("mm/filemap: map entire large folio faultaround")
> Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
> Reported-by: "Darrick J. Wong" <djwong@...nel.org>
> ---

Some of the code in here might deserve some cleanups IMHO :)

[...]

>   	addr0 = addr - start * PAGE_SIZE;
>   	if (folio_within_vma(folio, vmf->vma) &&
> -	    (addr0 & PMD_MASK) == ((addr0 + folio_size(folio) - 1) & PMD_MASK)) {
> +	    (addr0 & PMD_MASK) == ((addr0 + folio_size(folio) - 1) & PMD_MASK) &&

Isn't this just testing whether addr0 is aligned to folio_size(folio)? 
(given that we don't support folios > PMD_SIZE), like

	IS_ALIGNED(addr0, folio_size(folio))

Anyhow, unrelated to this patch ...

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb