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Thu, 30 Oct 2025 16:25:59 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id rF/mOJeRA2m3FgAAD6G6ig (envelope-from ); Thu, 30 Oct 2025 16:25:59 +0000 Date: Thu, 30 Oct 2025 16:25:54 +0000 From: Pedro Falcato To: Lorenzo Stoakes Cc: Andrew Morton , Jonathan Corbet , David Hildenbrand , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jann Horn , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Andrei Vagin Subject: Re: [PATCH 2/3] mm: implement sticky, copy on fork VMA flags Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 0C35540006 X-Stat-Signature: huzndhffii49rzxaa4s1jmeoq8r6nct9 X-HE-Tag: 1761841563-658100 X-HE-Meta: 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 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 Wed, Oct 29, 2025 at 04:50:32PM +0000, Lorenzo Stoakes wrote: > It's useful to be able to force a VMA to be copied on fork outside of the > parameters specified by vma_needs_copy(), which otherwise only copies page > tables if: > > * The destination VMA has VM_UFFD_WP set > * The mapping is a PFN or mixed map > * The mapping is anonymous and forked in (i.e. vma->anon_vma is non-NULL) > > Setting this flag implies that the page tables mapping the VMA are such > that simply re-faulting the VMA will not re-establish them in identical > form. > > We introduce VM_COPY_ON_FORK to clearly identify which flags require this > behaviour, which currently is only VM_MAYBE_GUARD. Do we want this to be sticky though? If you're looking for more granularity with this flag, the best option might be to stop merges from happening there. If not, I can imagine a VMA that merges with other VMAs far past the original guarded range, and thus you get no granularity (possibly, not even useful). If you're _not_ looking for granularity, then maybe using a per-mm flag for guard ranges or some other solution would be superior? The rest of the patch (superficially) looks good to me, though. -- Pedro