linux-kernel - Re: [PATCH 2/2] mm/truncate: Unmap large folio on split failure

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Message-ID: <37ceab54-c4b2-449e-aa46-ffaefe525737@redhat.com>
Date: Tue, 21 Oct 2025 11:47:11 +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>, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 Kiryl Shutsemau <kas@...nel.org>
Subject: Re: [PATCH 2/2] mm/truncate: Unmap large folio on split failure

On 21.10.25 11:44, David Hildenbrand wrote:
> On 21.10.25 08:35, 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.
>>
>> This behavior might not be respected on truncation.
>>
>> During truncation, the kernel splits a large folio in order to reclaim
>> memory. As a side effect, it unmaps the folio and destroys PMD mappings
>> of the folio. The folio will be refaulted as PTEs and SIGBUS semantics
>> are preserved.
>>
>> However, if the split fails, PMD mappings are preserved and the user
>> will not receive SIGBUS on any accesses within the PMD.
>>
>> Unmap the folio on split failure. It will lead to refault as PTEs and
>> preserve SIGBUS semantics.
> 
> Was the discussion on the old patch set already done? I can spot that
> you send this series 20min after asking Christoph

^ Dave

Also, please send a proper patch series including cover letter that 
describes the changes since the last RFC.

-- 
Cheers

David / dhildenb