linux-kernel - Re: [PATCHv2 1/2] mm/memory: Do not populate page table entries beyond i

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Message-ID: <96102837-402d-c671-1b29-527f2b5361bf@google.com>
Date: Mon, 27 Oct 2025 01:20:42 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Kiryl Shutsemau <kirill@...temov.name>
cc: Andrew Morton <akpm@...ux-foundation.org>, 
    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 1/2] mm/memory: Do not populate page table entries
 beyond i_size

On Thu, 23 Oct 2025, 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.
> 
> 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;

Sorry for coming in late as usual, and complicating matters.

> 
> 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")

ACK to restoring the correct POSIX behaviour to those filesystems
which are being given large folios beyond EOF transparently,
without any huge= mount option to permit it.

> Fixes: 800d8c63b2e9 ("shmem: add huge pages support")

But NAK to regressing the intentional behaviour of huge=always
on shmem/tmpfs: the page size, whenever possible, is PMD-sized.  In
6.18-rc huge=always is currently (thanks to Baolin) behaving correctly
again, as it had done for nine years: I insist we do not re-break it.

Andrew, please drop this version (and no need to worry about backports).

I'm guessing that yet another ugly shmem_file() or shmem_mapping()
exception should be good enough - I doubt you need to consider the
huge= option, just go by whether there is a huge folio already there -
though that would have an implication for the following patch.

(But what do I mean by "huge folio" above?  Do I mean large or do
I mean pmd_mappable?  It's the huge=always pmd_mappable folios I
care not to break, the mTHPy ones can be argued either way.)

Hugh