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[2003:d8:2f0d:ba00:c951:31d7:b2b0:8ba0]) by smtp.gmail.com with ESMTPSA id f6-20020adff586000000b00228c375d81bsm631245wro.2.2022.09.05.00.59.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 05 Sep 2022 00:59:48 -0700 (PDT) Message-ID: Date: Mon, 5 Sep 2022 09:59:47 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.2.0 To: John Hubbard , Yang Shi , peterx@redhat.com, kirill.shutemov@linux.intel.com, jgg@nvidia.com, hughd@google.com, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20220901222707.477402-1-shy828301@gmail.com> Content-Language: en-US From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05.09.22 00:29, John Hubbard wrote: > On 9/1/22 15:27, Yang Shi wrote: >> Since general RCU GUP fast was introduced in commit 2667f50e8b81 ("mm: >> introduce a general RCU get_user_pages_fast()"), a TLB flush is no longer >> sufficient to handle concurrent GUP-fast in all cases, it only handles >> traditional IPI-based GUP-fast correctly. On architectures that send >> an IPI broadcast on TLB flush, it works as expected. But on the >> architectures that do not use IPI to broadcast TLB flush, it may have >> the below race: >> >> CPU A CPU B >> THP collapse fast GUP >> gup_pmd_range() <-- see valid pmd >> gup_pte_range() <-- work on pte >> pmdp_collapse_flush() <-- clear pmd and flush >> __collapse_huge_page_isolate() >> check page pinned <-- before GUP bump refcount >> pin the page >> check PTE <-- no change >> __collapse_huge_page_copy() >> copy data to huge page >> ptep_clear() >> install huge pmd for the huge page >> return the stale page >> discard the stale page > > Hi Yang, > > Thanks for taking the trouble to write down these notes. I always > forget which race we are dealing with, and this is a great help. :) > > More... > >> >> The race could be fixed by checking whether PMD is changed or not after >> taking the page pin in fast GUP, just like what it does for PTE. If the >> PMD is changed it means there may be parallel THP collapse, so GUP >> should back off. >> >> Also update the stale comment about serializing against fast GUP in >> khugepaged. >> >> Fixes: 2667f50e8b81 ("mm: introduce a general RCU get_user_pages_fast()") >> Signed-off-by: Yang Shi >> --- >> mm/gup.c | 30 ++++++++++++++++++++++++------ >> mm/khugepaged.c | 10 ++++++---- >> 2 files changed, 30 insertions(+), 10 deletions(-) >> >> diff --git a/mm/gup.c b/mm/gup.c >> index f3fc1f08d90c..4365b2811269 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -2380,8 +2380,9 @@ static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start, >> } >> >> #ifdef CONFIG_ARCH_HAS_PTE_SPECIAL >> -static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, >> - unsigned int flags, struct page **pages, int *nr) >> +static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, >> + unsigned long end, unsigned int flags, >> + struct page **pages, int *nr) >> { >> struct dev_pagemap *pgmap = NULL; >> int nr_start = *nr, ret = 0; >> @@ -2423,7 +2424,23 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, >> goto pte_unmap; >> } >> >> - if (unlikely(pte_val(pte) != pte_val(*ptep))) { >> + /* >> + * THP collapse conceptually does: >> + * 1. Clear and flush PMD >> + * 2. Check the base page refcount >> + * 3. Copy data to huge page >> + * 4. Clear PTE >> + * 5. Discard the base page >> + * >> + * So fast GUP may race with THP collapse then pin and >> + * return an old page since TLB flush is no longer sufficient >> + * to serialize against fast GUP. >> + * >> + * Check PMD, if it is changed just back off since it >> + * means there may be parallel THP collapse. >> + */ > > As I mentioned in the other thread, it would be a nice touch to move > such discussion into the comment header. > >> + if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) || >> + unlikely(pte_val(pte) != pte_val(*ptep))) { > > > That should be READ_ONCE() for the *pmdp and *ptep reads. Because this > whole lockless house of cards may fall apart if we try reading the > page table values without READ_ONCE(). I came to the conclusion that the implicit memory barrier when grabbing a reference on the page is sufficient such that we don't need READ_ONCE here. If we still intend to change that code, we should fixup all GUP-fast functions in a similar way. But again, I don't think we need a change here. >> - * After this gup_fast can't run anymore. This also removes >> - * any huge TLB entry from the CPU so we won't allow >> - * huge and small TLB entries for the same virtual address >> - * to avoid the risk of CPU bugs in that area. >> + * This removes any huge TLB entry from the CPU so we won't allow >> + * huge and small TLB entries for the same virtual address to >> + * avoid the risk of CPU bugs in that area. >> + * >> + * Parallel fast GUP is fine since fast GUP will back off when >> + * it detects PMD is changed. >> */ >> _pmd = pmdp_collapse_flush(vma, address, pmd); > > To follow up on David Hildenbrand's note about this in the nearby thread... > I'm also not sure if pmdp_collapse_flush() implies a memory barrier on > all arches. It definitely does do an atomic op with a return value on x86, > but that's just one arch. > I think a ptep/pmdp clear + TLB flush really has to imply a memory barrier, otherwise TLB flushing code might easily mess up with surrounding code. But we should better double-check. s390x executes an IDTE instruction, which performs serialization (-> memory barrier). arm64 seems to use DSB instructions to enforce memory ordering. -- Thanks, David / dhildenb