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[70.27.3.10]) by smtp.gmail.com with ESMTPSA id h1-20020a05620a400100b006bbe7ded98csm554339qko.112.2022.08.26.14.37.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 26 Aug 2022 14:37:14 -0700 (PDT) Date: Fri, 26 Aug 2022 17:37:12 -0400 From: Peter Xu To: David Hildenbrand Cc: Alistair Popple , linux-mm@kvack.org, akpm@linux-foundation.org, Nadav Amit , huang ying , LKML , "Sierra Guiza, Alejandro (Alex)" , Felix Kuehling , Jason Gunthorpe , John Hubbard , Ralph Campbell , Matthew Wilcox , Karol Herbst , Lyude Paul , Ben Skeggs , Logan Gunthorpe , paulus@ozlabs.org, linuxppc-dev@lists.ozlabs.org, stable@vger.kernel.org, Huang Ying Subject: Re: [PATCH v3 2/3] mm/migrate_device.c: Copy pte dirty bit to page Message-ID: References: <3b01af093515ce2960ac39bb16ff77473150d179.1661309831.git-series.apopple@nvidia.com> <8735dkeyyg.fsf@nvdebian.thelocal> <8735dj7qwb.fsf@nvdebian.thelocal> <72146725-3d70-0427-50d4-165283a5a85d@redhat.com> <140e7688-b66d-2f6d-fed8-e39da5045420@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <140e7688-b66d-2f6d-fed8-e39da5045420@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 26, 2022 at 06:46:02PM +0200, David Hildenbrand wrote: > On 26.08.22 17:55, Peter Xu wrote: > > On Fri, Aug 26, 2022 at 04:47:22PM +0200, David Hildenbrand wrote: > >>> To me anon exclusive only shows this mm exclusively owns this page. I > >>> didn't quickly figure out why that requires different handling on tlb > >>> flushs. Did I perhaps miss something? > >> > >> GUP-fast is the magic bit, we have to make sure that we won't see new > >> GUP pins, thus the TLB flush. > >> > >> include/linux/mm.h:gup_must_unshare() contains documentation. > > > > Hmm.. Shouldn't ptep_get_and_clear() (e.g., xchg() on x86_64) already > > guarantees that no other process/thread will see this pte anymore > > afterwards? > > You could have a GUP-fast thread that just looked up the PTE and is > going to pin the page afterwards, after the ptep_get_and_clear() > returned. You'll have to wait until that thread finished. IIUC the early tlb flush won't protect concurrent fast-gup from happening, but I think it's safe because fast-gup will check pte after pinning, so either: (1) fast-gup runs before ptep_get_and_clear(), then page_try_share_anon_rmap() will fail properly, or, (2) fast-gup runs during or after ptep_get_and_clear(), then fast-gup will see that either the pte is none or changed, then it'll fail the fast-gup itself. > > Another user that relies on this interaction between GUP-fast and TLB > flushing is for example mm/ksm.c:write_protect_page() > > There is a comment in there explaining the interaction a bit more detailed. > > Maybe we'll be able to handle this differently in the future (maybe once > this turns out to be an actual performance problem). Unfortunately, > mm->write_protect_seq isn't easily usable because we'd need have to make > sure we're the exclusive writer. > > > For now, it's not too complicated. For PTEs: > * try_to_migrate_one() already uses ptep_clear_flush(). > * try_to_unmap_one() already conditionally used ptep_clear_flush(). > * migrate_vma_collect_pmd() was the one case that didn't use it already > (and I wonder why it's different than try_to_migrate_one()). I'm not sure whether I fully get the point, but here one major difference is all the rest handles one page, so a tlb flush alongside with the pte clear sounds reasonable. Even if so try_to_unmap_one() was modified to use tlb batching, but then I see that anon exclusive made that batching conditional. I also have question there on whether we can keep using the tlb batching even with anon exclusive pages there. In general, I still don't see how stall tlb could affect anon exclusive pages on racing with fast-gup, because the only side effect of a stall tlb is unwanted page update iiuc, the problem is fast-gup doesn't even use tlb, afaict.. Thanks, -- Peter Xu