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Peter Anvin" , Jacob Pan , Jason Gunthorpe , jpoimboe@kernel.org, Joerg Roedel , juri.lelli@redhat.com, Kent Overstreet , kinseyho@google.com, "Kirill A. Shutemov" , lstoakes@gmail.com, mgorman@suse.de, mic@digikod.net, michael.christie@oracle.com, Ingo Molnar , mjguzik@gmail.com, "Michael S. Tsirkin" , Nicholas Piggin , "Peter Zijlstra (Intel)" , Petr Mladek , Rick P Edgecombe , Steven Rostedt , Suren Baghdasaryan , Thomas Gleixner , Uladzislau Rezki , vincent.guittot@linaro.org, vschneid@redhat.com Content-Transfer-Encoding: quoted-printable Message-Id: <08EFDEDB-7BBB-4D9C-B7E5-D7370EC609BE@gmail.com> References: <20240311164638.2015063-1-pasha.tatashin@soleen.com> <20240311164638.2015063-12-pasha.tatashin@soleen.com> <3e180c07-53db-4acb-a75c-1a33447d81af@app.fastmail.com> To: Andy Lutomirski X-Mailer: Apple Mail (2.3774.400.31) X-Rspamd-Queue-Id: 5DAAE1C000E X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: eqeh6rg5zorg97hbuo3sqws9zatxpgad X-HE-Tag: 1710201390-727992 X-HE-Meta: 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 GUJBTv9s RkUODUuCOYqTA/96oYRTLKihmICQfA0gurRjkpqP5cilaAPai+zPCZVt278TgiFNMpo+5Nb5C+a0p5po8kzFrU5JL+c/HozAkqU6bogufi/DaZ/RHxbdeFpUc/8pZiyfGHtk+y/xbCv4bDAXJykE0fLQJrCb8IBW5svMxjBD+M6wacL8= 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 12 Mar 2024, at 1:41, Andy Lutomirski wrote: >=20 > On Mon, Mar 11, 2024, at 4:34 PM, Dave Hansen wrote: >> On 3/11/24 15:17, Andy Lutomirski wrote: >>> I *think* that all x86 implementations won't fill the TLB for a >>> non-accessed page without also setting the accessed bit, >>=20 >> That's my understanding as well. The SDM is a little more obtuse = about it: >>=20 >>> Whenever the processor uses a paging-structure entry as part of >>> linear-address translation, it sets the accessed flag in that entry >>> (if it is not already set). >>=20 >> but it's there. >>=20 >> But if we start needing Accessed=3D1 to be accurate, clearing those = PTEs >> gets more expensive because it needs to be atomic to lock out the = page >> walker. It basically needs to start getting treated similarly to = what >> is done for Dirty=3D1 on userspace PTEs. Not the end of the world, = of >> course, but one more source of overhead. >=20 > In my fantasy land where I understand the x86 paging machinery, = suppose we're in finish_task_switch(), and suppose prev is Not Horribly = Buggy (TM). In particular, suppose that no other CPU is concurrently = (non-speculatively!) accessing prev's stack. Prev can't be running, = because whatever magic lock prevents it from being migrated hasn't been = released yet. (I have no idea what lock this is, but it had darned well = better exist so prev isn't migrated before switch_to() even returns.) >=20 > So the current CPU is not accessing the memory, and no other CPU is = accessing the memory, and BPF doesn't exist, so no one is being utterly = daft and a kernel read probe, and perf isn't up to any funny business, = etc. And a CPU will never *speculatively* set the accessed bit (I told = you it's fantasy land), so we just do it unlocked: >=20 > if (!pte->accessed) { > *pte =3D 0; > reuse the memory; > } >=20 > What could possibly go wrong? >=20 > I admit this is not the best idea I've ever had, and I will not waste = anyone's time by trying very hard to defend it :) >=20 Just a thought: you don=E2=80=99t care if someone only reads from the = stack's page (you can just install another page later). IOW: you only = care if someone writes. So you can look on the dirty-bit, which is not being set speculatively = and save yourself one problem.