linux-kernel - Re: [PATCHv3 1/2] mm/memory: Do not populate page table entries beyond i

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Message-ID: <hw5hjbmt65aefgfz5cqsodpduvlkc6fmlbmwemvoknuehhgml2@orbho2mz52sv>
Date: Tue, 28 Oct 2025 10:23:31 +0000
From: Kiryl Shutsemau <kirill@...temov.name>
To: Andrew Morton <akpm@...ux-foundation.org>
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
Subject: Re: [PATCHv3 1/2] mm/memory: Do not populate page table entries
 beyond i_size

On Mon, Oct 27, 2025 at 03:33:23PM -0700, Andrew Morton wrote:
> On Mon, 27 Oct 2025 11:56:35 +0000 Kiryl Shutsemau <kirill@...temov.name> 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;
> > 
> > Make an exception for shmem/tmpfs that for long time intentionally
> > mapped with PMDs 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: 01c70267053d ("fs: add a filesystem flag for THPs")
> 
> Multiple Fixes: are confusing.
> 
> We have two 6.18-rcX targets and one from 2020.  Are we asking people
> to backport this all the way back to 2020?  If so I'd suggest the
> removal of the more recent Fixes: targets.

Okay, fair enough.

> Also, is [2/2] to be backported?  The changelog makes it sound that way,
> but no Fixes: was identified?

Looking at split-on-truncate history, looks like this is the right
commit to point to:

Fixes: b9a8a4195c7d ("truncate,shmem: Handle truncates that split large folios")

It moves split logic from shmem-specific to generic truncate.

As with the first patch, it will not be a trivial backport, but I am
around to help with this.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov