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(p200300cbc706740083daebadba7fc97c.dip0.t-ipconnect.de. [2003:cb:c706:7400:83da:ebad:ba7f:c97c]) by smtp.gmail.com with ESMTPSA id d4-20020adffd84000000b003141f96ed36sm16156605wrr.0.2023.08.22.08.28.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 22 Aug 2023 08:28:09 -0700 (PDT) Message-ID: Date: Tue, 22 Aug 2023 17:28:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped_thp() versus uffd To: Jann Horn , Hugh Dickins Cc: Andrew Morton , Mike Kravetz , Mike Rapoport , "Kirill A. Shutemov" , Matthew Wilcox , Suren Baghdasaryan , Qi Zheng , Yang Shi , Mel Gorman , Peter Xu , Peter Zijlstra , Will Deacon , Yu Zhao , Alistair Popple , Ralph Campbell , Ira Weiny , Steven Price , SeongJae Park , Lorenzo Stoakes , Huang Ying , Naoya Horiguchi , Christophe Leroy , Zack Rusin , Jason Gunthorpe , Axel Rasmussen , Anshuman Khandual , Pasha Tatashin , Miaohe Lin , Minchan Kim , Christoph Hellwig , Song Liu , Thomas Hellstrom , Russell King , "David S. Miller" , Michael Ellerman , "Aneesh Kumar K.V" , Heiko Carstens , Christian Borntraeger , Claudio Imbrenda , Alexander Gordeev , Gerald Schaefer , Vasily Gorbik , Vishal Moola , Vlastimil Babka , Zi Yan , Zach O'Keefe , Linux ARM , sparclinux@vger.kernel.org, linuxppc-dev , linux-s390 , kernel list , Linux-MM References: <4d31abf5-56c0-9f3d-d12f-c9317936691@google.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: DAD4520012 X-Stat-Signature: jji7xmbn9nbsdkgt6333j396msodypha X-HE-Tag: 1692718093-486904 X-HE-Meta: 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 idtl+wOK 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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: On 22.08.23 16:39, Jann Horn wrote: > On Tue, Aug 22, 2023 at 4:51 AM Hugh Dickins wrote: >> On Mon, 21 Aug 2023, Jann Horn wrote: >>> On Mon, Aug 21, 2023 at 9:51 PM Hugh Dickins wrote: >>>> Just for this case, take the pmd_lock() two steps earlier: not because >>>> it gives any protection against this case itself, but because ptlock >>>> nests inside it, and it's the dropping of ptlock which let the bug in. >>>> In other cases, continue to minimize the pmd_lock() hold time. >>> >>> Special-casing userfaultfd like this makes me a bit uncomfortable; but >>> I also can't find anything other than userfaultfd that would insert >>> pages into regions that are khugepaged-compatible, so I guess this >>> works? >> >> I'm as sure as I can be that it's solely because userfaultfd breaks >> the usual rules here (and in fairness, IIRC Andrea did ask my permission >> before making it behave that way on shmem, COWing without a source page). >> >> Perhaps something else will want that same behaviour in future (it's >> tempting, but difficult to guarantee correctness); for now, it is just >> userfaultfd (but by saying "_armed" rather than "_missing", I'm half- >> expecting uffd to add more such exceptional modes in future). > > Hm, yeah, sounds okay. (I guess we'd also run into this if we ever > wanted to make it possible to reliably install PTE markers with > madvise() or something like that, which might be nice for allowing > userspace to create guard pages without unnecessary extra VMAs...) I'm working on something similar that goes a bit further than just guard pages. It also installs PTE markers into page tables, inside existing large VMAs. Initially, I'll only tackle anon VMAs, though. -- Cheers, David / dhildenb