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[2003:cb:c706:2900:1613:4308:aca3:2786]) by smtp.gmail.com with ESMTPSA id u21-20020a7bc055000000b003a3442f1229sm1229908wmc.29.2022.08.31.00.44.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 31 Aug 2022 00:44:57 -0700 (PDT) Message-ID: Date: Wed, 31 Aug 2022 09:44:56 +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: Jason Gunthorpe Cc: John Hubbard , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Mel Gorman , "Matthew Wilcox (Oracle)" , Andrea Arcangeli , Hugh Dickins , Peter Xu References: <20220825164659.89824-1-david@redhat.com> <20220825164659.89824-3-david@redhat.com> <1892f6de-fd22-0e8b-3ff6-4c8641e1c68e@redhat.com> <2e20c90d-4d1f-dd83-aa63-9d8d17021263@redhat.com> <9ce3aaaa-71a6-5a81-16a3-36e6763feb91@redhat.com> <9a4fe603-950e-785b-6281-2e309256463f@nvidia.com> <68b38ac4-c680-b694-21a9-1971396d63b9@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v1 2/3] mm/gup: use gup_can_follow_protnone() also in GUP-fast 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 31.08.22 01:44, Jason Gunthorpe wrote: > On Tue, Aug 30, 2022 at 09:23:44PM +0200, David Hildenbrand wrote: >> @@ -2997,6 +2997,11 @@ static inline bool gup_must_unshare(unsigned int flags, struct page *page) >> */ >> if (!PageAnon(page)) >> return false; >> + >> + /* See page_try_share_anon_rmap() for GUP-fast details. */ >> + if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && irqs_disabled()) >> + smp_rmb(); >> + >> /* >> * Note that PageKsm() pages cannot be exclusive, and consequently, >> * cannot get pinned. >> diff --git a/include/linux/rmap.h b/include/linux/rmap.h >> index bf80adca980b..454c159f2aae 100644 >> --- a/include/linux/rmap.h >> +++ b/include/linux/rmap.h >> @@ -267,7 +267,7 @@ static inline int page_try_dup_anon_rmap(struct page *page, bool compound, >> * @page: the exclusive anonymous page to try marking possibly shared >> * >> * The caller needs to hold the PT lock and has to have the page table entry >> - * cleared/invalidated+flushed, to properly sync against GUP-fast. >> + * cleared/invalidated. >> * >> * This is similar to page_try_dup_anon_rmap(), however, not used during fork() >> * to duplicate a mapping, but instead to prepare for KSM or temporarily >> @@ -283,12 +283,60 @@ static inline int page_try_share_anon_rmap(struct page *page) >> { >> VM_BUG_ON_PAGE(!PageAnon(page) || !PageAnonExclusive(page), page); >> >> - /* See page_try_dup_anon_rmap(). */ >> - if (likely(!is_device_private_page(page) && >> - unlikely(page_maybe_dma_pinned(page)))) >> - return -EBUSY; >> + /* device private pages cannot get pinned via GUP. */ >> + if (unlikely(is_device_private_page(page))) { >> + ClearPageAnonExclusive(page); >> + return 0; >> + } >> >> + /* >> + * We have to make sure that while we clear PageAnonExclusive, that >> + * the page is not pinned and that concurrent GUP-fast won't succeed in >> + * concurrently pinning the page. >> + * >> + * Conceptually, GUP-fast pinning code of anon pages consists of: >> + * (1) Read the PTE >> + * (2) Pin the mapped page >> + * (3) Check if the PTE changed by re-reading it; back off if so. >> + * (4) Check if PageAnonExclusive is not set; back off if so. >> + * >> + * Conceptually, PageAnonExclusive clearing code consists of: >> + * (1) Clear PTE >> + * (2) Check if the page is pinned; back off if so. >> + * (3) Clear PageAnonExclusive >> + * (4) Restore PTE (optional) >> + * >> + * In GUP-fast, we have to make sure that (2),(3) and (4) happen in >> + * the right order. Memory order between (2) and (3) is handled by >> + * GUP-fast, independent of PageAnonExclusive. >> + * >> + * When clearing PageAnonExclusive(), we have to make sure that (1), >> + * (2), (3) and (4) happen in the right order. >> + * >> + * Note that (4) has to happen after (3) in both cases to handle the >> + * corner case whereby the PTE is restored to the original value after >> + * clearing PageAnonExclusive and while GUP-fast might not detect the >> + * PTE change, it will detect the PageAnonExclusive change. >> + * >> + * We assume that there might not be a memory barrier after >> + * clearing/invalidating the PTE (1) and before restoring the PTE (4), >> + * so we use explicit ones here. >> + * >> + * These memory barriers are paired with memory barriers in GUP-fast >> + * code, including gup_must_unshare(). >> + */ >> + >> + /* Clear/invalidate the PTE before checking for PINs. */ >> + if (IS_ENABLED(CONFIG_HAVE_FAST_GUP)) >> + smp_mb(); >> + >> + if (unlikely(page_maybe_dma_pinned(page))) >> + return -EBUSY; > > It is usually a bad sign to see an attempt to create a "read release".. I still have to get used to the acquire/release semantics ... :) > >> ClearPageAnonExclusive(page); >> + >> + /* Clear PageAnonExclusive() before eventually restoring the PTE. */ >> + if (IS_ENABLED(CONFIG_HAVE_FAST_GUP)) >> + smp_mb__after_atomic(); >> return 0; >> } > > I don't know enough about the memory model to say if this is OK.. I guess it's best to include some memory model folks once we have something that looks reasonable. > > Generally, I've never seen an algorithm be successfull with these > kinds of multi-atomic gyrations. Yeah, I'm absolutely looking for a nicer alternative to sync with RCU GUP-fast. So far I wasn't successful. > > If we break it down a bit, and replace the 'read release' with an > actual atomic for discussion: > > > CPU0 CPU1 > clear pte > incr_return ref // release & acquire > add_ref // acquire > > This seems OK, if CPU1 views !dma then CPU0 must view clear pte due to > the atomic's release/acquire semantic > > If CPU1 views dma then it just exits > > > Now the second phase: > > CPU0 CPU1 > clear anon_exclusive > restore pte // release > > read_pte // acquire > read anon_exclusive > > If CPU0 observes the restored PTE then it must observe the cleared > anon_exclusive > > Otherwise CPU0 must observe the cleared PTE. > > So, maybe I could convince myself it is OK, but I think your placement > of barriers is confusing as to what data the barrier is actually > linked to. > > We are using a barrier around the ref - acquire on the CPU0 and full > barier on the CPU1 (eg atomic_read(); smb_mb_after_atomic() ) When dropping the other patch, I think we still need something like diff --git a/mm/gup.c b/mm/gup.c index 5abdaf487460..8c5ff41d5e56 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -158,6 +158,13 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags) else folio_ref_add(folio, refs * (GUP_PIN_COUNTING_BIAS - 1)); + /* + * Adjust the pincount before re-checking the PTE for changes. + * + * Paired with a memory barrier in page_try_share_anon_rmap(). + */ + smb_mb__after_atomic(); + node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, refs); return folio; > > The second phase uses a smp_store_release/load_acquire on the PTE. > > It is the same barriers you sketched but integrated with the data they > are ordering. Sorry for having to ask, but what exactly would be your suggestion? Thanks for having a look! -- Thanks, David / dhildenb