linux-kernel - Re: [PATCHv2 2/2] mm/truncate: Unmap large folio on split failure

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Message-Id: <20251023135644.f955b3aa4b4df23f621087c4@linux-foundation.org>
Date: Thu, 23 Oct 2025 13:56:44 -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 2/2] mm/truncate: Unmap large folio on split failure

On Thu, 23 Oct 2025 10:32:51 +0100 Kiryl Shutsemau <kirill@...temov.name> wrote:

> 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.

This conflicts significantly with mm-hotfixes's
https://lore.kernel.org/all/20251017013630.139907-1-ziy@nvidia.com/T/#u,
whcih is cc:stable.

What do do here?