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([2600:1010:b015:9bd0:bda1:66ef:aea3:7e99]) by smtp.gmail.com with ESMTPSA id 73sm152935pga.26.2021.01.12.16.31.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 12 Jan 2021 16:31:42 -0800 (PST) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable From: Andy Lutomirski Mime-Version: 1.0 (1.0) Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect Date: Tue, 12 Jan 2021 16:31:41 -0800 Message-Id: <68095DFB-7D72-480E-BDF3-3C88B8428867@amacapital.net> References: Cc: Will Deacon , Yu Zhao , Laurent Dufour , Peter Zijlstra , Vinayak Menon , Linus Torvalds , Andy Lutomirski , Peter Xu , Andrea Arcangeli , linux-mm , lkml , Pavel Emelyanov , Mike Kravetz , Mike Rapoport , stable , Minchan Kim , surenb@google.com In-Reply-To: To: Nadav Amit X-Mailer: iPhone Mail (18C66) 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 Jan 12, 2021, at 2:29 PM, Nadav Amit wrote: >=20 > =EF=BB=BF >>=20 >> On Jan 12, 2021, at 1:43 PM, Will Deacon wrote: >>=20 >> On Tue, Jan 12, 2021 at 12:38:34PM -0800, Nadav Amit wrote: >>>> On Jan 12, 2021, at 11:56 AM, Yu Zhao wrote: >>>>> On Tue, Jan 12, 2021 at 11:15:43AM -0800, Nadav Amit wrote: >>>>>> I will send an RFC soon for per-table deferred TLB flushes tracking. >>>>>> The basic idea is to save a generation in the page-struct that tracks= >>>>>> when deferred PTE change took place, and track whenever a TLB flush >>>>>> completed. In addition, other users - such as mprotect - would use >>>>>> the tlb_gather interface. >>>>>>=20 >>>>>> Unfortunately, due to limited space in page-struct this would only >>>>>> be possible for 64-bit (and my implementation is only for x86-64). >>>>>=20 >>>>> I don't want to discourage you but I don't think this would end up >>>>> well. PPC doesn't necessarily follow one-page-struct-per-table rule, >>>>> and I've run into problems with this before while trying to do >>>>> something similar. >>>=20 >>> Discourage, discourage. Better now than later. >>>=20 >>> It will be relatively easy to extend the scheme to be per-VMA instead of= >>> per-table for architectures that prefer it this way. It does require >>> TLB-generation tracking though, which Andy only implemented for x86, so I= >>> will focus on x86-64 right now. >>=20 >> Can you remind me of what we're missing on arm64 in this area, please? I'= m >> happy to help get this up and running once you have something I can build= >> on. >=20 > Let me first finish making something that we can use as a basis for a > discussion. I do not waste your time before I have something ready. If you want a hand, let me know. I suspect you understand the x86 code as w= ell as I do at this point, though :)