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Tue, 21 Oct 2025 07:31:12 -0400 (EDT) Date: Tue, 21 Oct 2025 12:31:10 +0100 From: Kiryl Shutsemau To: David Hildenbrand Cc: Andrew Morton , Hugh Dickins , Matthew Wilcox , Alexander Viro , Christian Brauner , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Rik van Riel , Harry Yoo , Johannes Weiner , Shakeel Butt , Baolin Wang , "Darrick J. Wong" , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] mm/truncate: Unmap large folio on split failure Message-ID: References: <20251021063509.1101728-1-kirill@shutemov.name> <20251021063509.1101728-2-kirill@shutemov.name> <37ceab54-c4b2-449e-aa46-ffaefe525737@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <37ceab54-c4b2-449e-aa46-ffaefe525737@redhat.com> X-Rspamd-Server: rspam01 X-Stat-Signature: tt7u8bmndju59syhz78j48tr8175n6co X-Rspam-User: X-Rspamd-Queue-Id: D3A7F140014 X-HE-Tag: 1761046276-703823 X-HE-Meta: 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 daeg4xCQ zHohHULpY4dtwn1e6pIAybTfEy+pfY4QZObIGpI8edossQr6N/N8YkT4tqvkdi29Iry5n6gI4MU5/Ri8cAt/uX7NFX6ZeBiIfgfGvGjHjUq/CYm5+GPgqrONX8NpGSSkN0+avgDqc7E8lqNzAOpanx/BeYxkb+qRG+9aba17MKy8uSqaAT6PgATAj5hNBu9qsduWGc3fppK5qzrZVER4K2vDA59mCDw2sw/ihNhcC+m4lQUf+6nQyQ8EO2BR/JzIZRPGRAGBJd29TSMVugQZWLOrUW/d+X6yKxr4klbRnxj+QQDhJu0r78wEbP2ZS6YATTiFim4S1xC6V140pF920YGSjjb80+1s64KJMfymbp3g6OtanBqFx9tCEEjy47Zn5Fk/jhM6Qna3YM4W+fdzX1DkIDjfssa/D19FiUJPr/VTMZQ01YjkTu8jdohQa1xl7/wJQImQrLmGDUkooLMa8o3zMFA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > > > > > > 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