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Message-ID: <b1144819-2d03-4e63-b0e7-0fda6130de7d@lucifer.local> Date: Tue, 2 May 2023 20:37:04 +0100 From: Lorenzo Stoakes <lstoakes@...il.com> To: David Hildenbrand <david@...hat.com> Cc: Jason Gunthorpe <jgg@...dia.com>, Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>, Matthew Wilcox <willy@...radead.org>, Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>, Leon Romanovsky <leon@...nel.org>, Christian Benvenuti <benve@...co.com>, Nelson Escobar <neescoba@...co.com>, Bernard Metzler <bmt@...ich.ibm.com>, Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Bjorn Topel <bjorn@...nel.org>, Magnus Karlsson <magnus.karlsson@...el.com>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Jonathan Lemon <jonathan.lemon@...il.com>, "David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Christian Brauner <brauner@...nel.org>, Richard Cochran <richardcochran@...il.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org, netdev@...r.kernel.org, bpf@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>, John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>, "Kirill A . Shutemov" <kirill@...temov.name>, Pavel Begunkov <asml.silence@...il.com>, Mika Penttila <mpenttil@...hat.com>, Dave Chinner <david@...morbit.com>, Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>, Matthew Rosato <mjrosato@...ux.ibm.com>, "Paul E . McKenney" <paulmck@...nel.org>, Christian Borntraeger <borntraeger@...ux.ibm.com>, Mike Rapoport <rppt@...ux.ibm.com> Subject: Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings On Tue, May 02, 2023 at 09:33:45PM +0200, David Hildenbrand wrote: > On 02.05.23 21:25, Lorenzo Stoakes wrote: > > On Tue, May 02, 2023 at 04:07:50PM -0300, Jason Gunthorpe wrote: > > > On Tue, May 02, 2023 at 07:17:14PM +0100, Lorenzo Stoakes wrote: > > > > > > > On a specific point - if mapping turns out to be NULL after we confirm > > > > stable PTE, I'd be inclined to reject and let the slow path take care of > > > > it, would you agree that that's the correct approach? > > > > > > I think in general if GUP fast detects any kind of race it should bail > > > to the slow path. > > > > > > The races it tries to resolve itself should have really safe and > > > obvious solutions. > > > > > > I think this comment is misleading: > > > > > > > + /* > > > > + * GUP-fast disables IRQs - this prevents IPIs from causing page tables > > > > + * to disappear from under us, as well as preventing RCU grace periods > > > > + * from making progress (i.e. implying rcu_read_lock()). > > > > > > True, but that is not important here since we are not reading page > > > tables > > > > > > > + * This means we can rely on the folio remaining stable for all > > > > + * architectures, both those that set CONFIG_MMU_GATHER_RCU_TABLE_FREE > > > > + * and those that do not. > > > > > > Not really clear. We have a valid folio refcount here, that is all. > > > > Some of this is a product of mixed signals from different commenters and > > my being perhaps a little _too_ willing to just go with the flow. > > > > With interrupts disabled and IPI blocked, plus the assurances that > > interrupts being disabled implied the RCU version of page table > > manipulation is also blocked, my understanding was that remapping in this > > process to another page could not occur. > > > > Of course the folio is 'stable' in the sense we have a refcount on it, but > > it is unlocked so things can change. > > > > I'm guessing the RCU guarantees in the TLB logic are not as solid as IPI, > > because in the IPI case it seems to me you couldn't even clear the PTE > > entry before getting to the page table case. > > > > Otherwise, I'm a bit uncertain actually as to how we can get to the point > > where the folio->mapping is being manipulated. Is this why? > > I'll just stress again that I think there are cases where we unmap and free > a page without synchronizing against GUP-fast using an IPI or RCU. OK that explains why we need to be careful. Don't worry I am going to move the check after we confirm PTE entry hasn't changed. > > That's one of the reasons why we recheck if the PTE changed to back off, so > I've been told. > > I'm happy if someone proves me wrong and a page we just (temporarily) pinned > cannot have been freed+reused in the meantime. Let's play it safe for now :) > > -- > Thanks, > > David / dhildenb >
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