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

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Message-ID: <e4349d5a-33e8-4b8d-b1ad-6192ba00ff66@redhat.com>
Date: Tue, 21 Oct 2025 13:54:51 +0200
From: David Hildenbrand <david@...hat.com>
To: Kiryl Shutsemau <kirill@...temov.name>
Cc: 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>,
 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
Subject: Re: [PATCH 2/2] mm/truncate: Unmap large folio on split failure

On 21.10.25 13:31, Kiryl Shutsemau wrote:
> On Tue, Oct 21, 2025 at 11:47:11AM +0200, David Hildenbrand wrote:
>> 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 Dave
> 
> Based on feedback from Dave and Christoph on this patchset as well as
> comments form Matthew and Darrick ont the report thread I see that my
> idea to relax SIGBUS semantics for large folios will not fly :/

Then I was probably misreading the last email from you, likely the 
question you raised was independent of the progress of this series and 
more of general nature I assume.

> 
> But if you want to weigh in...

No, I think this makes sense. It's a regression that should be fixed.

> 
>> Also, please send a proper patch series including cover letter that
>> describes the changes since the last RFC.
> 
> There is no change besides Signed-off-bys.

Then point that out, please. It's common practice in MM to send cover 
letters for each new revision.

For example, Andrew will usually incorporate the cover letter into patch 
#1 when merging.

-- 
Cheers

David / dhildenb