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Tue, 03 Jun 2025 11:36:49 -0700 (PDT) MIME-Version: 1.0 References: <20250602210710.106159-1-lorenzo.stoakes@oracle.com> In-Reply-To: From: Jann Horn Date: Tue, 3 Jun 2025 20:36:13 +0200 X-Gm-Features: AX0GCFuBBAO2PAXs-lVLk6mYc2WXvqPy4BatY5BO3b03KdmWhPbCsmTQkGQ00QY Message-ID: Subject: Re: [PATCH] docs/mm: expand vma doc to highlight pte freeing, non-vma traversal To: Lorenzo Stoakes Cc: Andrew Morton , Suren Baghdasaryan , "Liam R . Howlett" , Vlastimil Babka , Shakeel Butt , Jonathan Corbet , Qi Zheng , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Stat-Signature: rgxmuj3abseishkp4jaohn7467wj6ciu X-Rspamd-Queue-Id: A10B3140010 X-Rspamd-Server: rspam11 X-HE-Tag: 1748975811-395649 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 Tue, Jun 3, 2025 at 12:45=E2=80=AFPM Lorenzo Stoakes wrote: > On Tue, Jun 03, 2025 at 12:25:36AM +0200, Jann Horn wrote: > > The one way in which I think this is currently kinda yolo/broken is > > that vmap_try_huge_pud() can end up freeing page tables via > > pud_free_pmd_page(), while holding no MM locks AFAICS, so that could > > race with the ptdump debug logic such that ptdump walks into freed > > page tables? > > But those mappings would be kernel mappings? How could ptdump walk into > those? /sys/kernel/debug/page_tables/kernel dumps kernel page tables. And I think /sys/kernel/debug/page_tables/current_kernel dumps page tables for the entire address space including both userspace and kernel. > > (I think we should take all the vma locks in that ptdump code and get > > rid of this weird exception instead of documenting it.) > > We really need to be sure that there aren't some weird architectures doin= g > weird things or circumstances where this is meaningful. > > I mean people went to great lengths to make this possible, I find it hard > to believe there aren't some _weird_ real world use cases. FWIW, looking through the git logs for the x86 version of it, it seems to mainly be used by developers of arch-specific code trying to debug/validate kernel behavior.