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[2003:cb:c70c:d200:fbf4:8c3c:56fa:173d]) by smtp.gmail.com with ESMTPSA id j2-20020a056000124200b0022537d826f3sm824156wrx.23.2022.08.26.15.19.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Aug 2022 15:19:51 -0700 (PDT) Message-ID: Date: Sat, 27 Aug 2022 00:19:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Content-Language: en-US To: Peter Xu 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 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> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v3 2/3] mm/migrate_device.c: Copy pte dirty bit to page In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 26.08.22 23:37, Peter Xu wrote: > 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. > Good that we're talking about it, very helpful! If that's actually not required -- good. What I learned how GUP-fast and TLB flushes interact is the following: GUP-fast disables local interrupts. A TLB flush will send an IPI and wait until it has been processed. This implies, that once the TLB flush succeeded, that the interrupt was handled and GUP-fast cannot be running anymore. BUT, there is the new RCU variant nowadays, and the TLB flush itself should not actually perform such a sync. They merely protect the page tables from getting freed and the THP from getting split IIUC. And you're correct that that wouldn't help. > 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. I think you're right and I might have managed to confuse myself with the write_protect_page() comment. I placed the gup_must_unshare() check explicitly after the "pte changed" check for this reason. So once the PTE was cleared, GUP-fast would undo any pin performed on a now-stale PTE. > >> >> 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.. I have the gut feeling that the comment in write_protect_page() is indeed stale, and that clearing PageAnonExclusive doesn't strictly need the TLB flush. I'll try to refresh my memory if there was any other case that I had to handle over the weekend. Thanks! -- Thanks, David / dhildenb