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[99.254.114.190]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6d3961dfad2sm86443686d6.1.2024.11.13.08.39.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 13 Nov 2024 08:39:33 -0800 (PST) Date: Wed, 13 Nov 2024 11:39:30 -0500 From: Peter Xu To: Jann Horn Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, "Kirill A . Shutemov" , Nicholas Piggin , David Hildenbrand , Matthew Wilcox , Andrew Morton , James Houghton , Huang Ying , "Aneesh Kumar K . V" , Vlastimil Babka , Rick P Edgecombe , Hugh Dickins , Borislav Petkov , Christophe Leroy , Michael Ellerman , Rik van Riel , Dan Williams , Mel Gorman , x86@kernel.org, Ingo Molnar , linuxppc-dev@lists.ozlabs.org, Dave Hansen , Dave Jiang , Oscar Salvador , Thomas Gleixner Subject: Re: [PATCH v5 0/7] mm/mprotect: Fix dax puds Message-ID: References: <20240812181225.1360970-1-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: RjnnKyJJaTnkwkKogK0szHAdkil9a0DmWQDDWk2XhaM_1731515974 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: B5E40A000B X-Stat-Signature: hjeioykj8gfpedjjx8y178kw3rsfmnz9 X-Rspam-User: X-HE-Tag: 1731515933-826554 X-HE-Meta: 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 JAKOaLt1 RMM/QGuS+g+armqH3Y+8zzgs4p8CXsbHLEa+PsjwrXsK3UxK/YBbg4WD9zif0nhr6UrXuUKDBgLqMZYKhdQIZEcOsfNuGhInD5gunHRLVOxzZdgf3grUtI0dNtopW5TGTJNONWkzc5QMDnmcyIDKjL9ffVRb/Rmco1gUdzS5jtimw0VLktcHAZUs8TvmJqAT/77n1huM13JDpGbX9cGZ37Z/0K5o6CjAb8gk8cPNq/U6eQhwn+YrgRQ6cOQLRW/0pD96oKXyu5NX2dYqivLEZ69PfAVtQnPbwXePt8W7Pm47IXR70WTRqRlRDgsA4LIaL2TnVxqtLocICHJHZMJyepBXPSOm97qn92pznrfw6TUUeM7m2VyqhCReC7W7Gu0os66nx632xwxzJg6+YuhVO8DUiSDMk5WUiy6L6e5Fx3koDk30= 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 Mon, Nov 11, 2024 at 10:20:59PM +0100, Jann Horn wrote: > On Mon, Aug 12, 2024 at 8:12 PM Peter Xu wrote: > > Dax supports pud pages for a while, but mprotect on puds was missing since > > the start. This series tries to fix that by providing pud handling in > > mprotect(). The goal is to add more types of pud mappings like hugetlb or > > pfnmaps. This series paves way for it by fixing known pud entries. > > Do people actually use hardware where they can use PUD THP mappings > for DAX? I thought that was just some esoteric feature that isn't > actually usable on almost any system. > Was I wrong about that? I did run it with a qemu emulated nvdimm device. Though in reality I've no idea on how many people are using it.. > > I think another example that probably doesn't play entirely nice with > PUD THP mappings is mremap()'s move_page_tables(). If > dax_get_unmapped_area() allows creating a VMA at an unaligned start > address (which I think it does?), move_page_tables() can probably end > up copying from an aligned address mapped with a huge PUD entry to an > unaligned address that needs to be mapped at the PTE level, and I > think that will probably cause it to call into get_old_pmd() while a > huge PUD entry is still present, which will probably get us a > pud_bad() error or such? I think you're probably right, that we have other places that may not work well with pud mappings. I also wonder whether dax_get_unmapped_area() needs to properly handle MAP_FIXED, even for PMD mappings. It looks like it always fallbacks to the default mm_get_unmapped_area() with FIXED, which have no idea on dax->alignment so it'll always allow it.. The issue is I'm not sure dax pmd can be split at all, while I think split-able is needed when mremap from a pmd-aligned address to a !pmd-aligned address. Thanks, -- Peter Xu