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

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Message-ID: <eokncpih37zm7ypt6gn5xyetx6jlemhvvfdzpmdlxleqlsqcr4@45h5w5ahwugs>
Date: Tue, 21 Oct 2025 12:31:10 +0100
From: Kiryl Shutsemau <kirill@...temov.name>
To: David Hildenbrand <david@...hat.com>
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 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 :/

But if you want to weigh in...

> 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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov