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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id u189-20020a0dd2c6000000b00559be540b56sm3639714ywd.134.2023.05.29.07.36.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 May 2023 07:36:52 -0700 (PDT) Date: Mon, 29 May 2023 07:36:40 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: Matthew Wilcox cc: Hugh Dickins , Andrew Morton , Mike Kravetz , Mike Rapoport , "Kirill A. Shutemov" , David Hildenbrand , 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 , 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 , Jann Horn , linux-arm-kernel@lists.infradead.org, sparclinux@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 05/12] powerpc: add pte_free_defer() for pgtables sharing page In-Reply-To: Message-ID: References: <35e983f5-7ed3-b310-d949-9ae8b130cdab@google.com> <28eb289f-ea2c-8eb9-63bb-9f7d7b9ccc11@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: A1691C0025 X-Stat-Signature: 88r43atnpr9b3e1dx3skh1cugd6zs3pw X-HE-Tag: 1685371013-15403 X-HE-Meta: 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 kOv83Ros srLuW54XQVXrVSyB+OhoDchTmIwiFFXZvTh6sEEem05o3co8AdBuOhIhqGjHg6UrfSzKP3SFVQ1Pee+AQmbfH3/Re2ajt9A9EDQuTMPWDnmGQXXAp09h5NRn80IpUbEVcop7zfO/6viAbsBU4+uxbf+fxNwYJ0amr891OmJ1OetWS+wtYZlQkciA8s4IRMvWuCuvHzVCMbhTl9NXWyCPKTrYOx2dkgMSPf1K2/zhNvEuL97M+bDgt7IGwJh6jENkfrjtYHkOhtxXfvtNAoBGNyR3j7qFjjZd2vkDCtKrXikAp9Ty0TrVeqbEpFj2EC4pjC1bUSgN7Xgg/7HL1WajlYIe0YpCIfu0W8BqMLQwi6g93zXuF+P3qV4RnKzVogSj78Il1u7em+h2bykndsJ/FjpHTZJxNXewrDFvg 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 Mon, 29 May 2023, Matthew Wilcox wrote: > On Sun, May 28, 2023 at 11:20:21PM -0700, Hugh Dickins wrote: > > +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable) > > +{ > > + struct page *page; > > + > > + page = virt_to_page(pgtable); > > + call_rcu(&page->rcu_head, pte_free_now); > > +} > > This can't be safe (on ppc). IIRC you might have up to 16x4k page > tables sharing one 64kB page. So if you have two page tables from the > same page being defer-freed simultaneously, you'll reuse the rcu_head > and I cannot imagine things go well from that point. Oh yes, of course, thanks for catching that so quickly. So my s390 and sparc implementations will be equally broken. > > I have no idea how to solve this problem. I do: I'll have to go back to the more complicated implementation we actually ran with on powerpc - I was thinking those complications just related to deposit/withdraw matters, forgetting the one-rcu_head issue. It uses large (0x10000) increments of the page refcount, avoiding call_rcu() when already active. It's not a complication I had wanted to explain or test for now, but we shall have to. Should apply equally well to sparc, but s390 more of a problem, since s390 already has its own refcount cleverness. Thanks, I must dash, out much of the day. Hugh