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[104.129.187.94]) by smtp.gmail.com with ESMTPSA id q2sm3656607pfg.144.2019.09.24.08.29.12 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 24 Sep 2019 08:29:29 -0700 (PDT) Subject: Re: [PATCH v8 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared To: Catalin Marinas , "Justin He (Arm Technology China)" Cc: Will Deacon , Mark Rutland , James Morse , Marc Zyngier , Matthew Wilcox , "Kirill A. Shutemov" , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , Suzuki Poulose , Punit Agrawal , Anshuman Khandual , Alex Van Brunt , Robin Murphy , Thomas Gleixner , Andrew Morton , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Ralph Campbell , "Kaly Xin (Arm Technology China)" , nd References: <20190921135054.142360-1-justin.he@arm.com> <20190921135054.142360-4-justin.he@arm.com> <20190923170433.GE10192@arrakis.emea.arm.com> <20190924103324.GB41214@arrakis.emea.arm.com> From: Jia He Message-ID: <6267b685-5162-85ac-087f-112303bb7035@gmail.com> Date: Tue, 24 Sep 2019 23:29:07 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20190924103324.GB41214@arrakis.emea.arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Catalin On 2019/9/24 18:33, Catalin Marinas wrote: > On Tue, Sep 24, 2019 at 06:43:06AM +0000, Justin He (Arm Technology China) wrote: >> Catalin Marinas wrote: >>> On Sat, Sep 21, 2019 at 09:50:54PM +0800, Jia He wrote: >>>> @@ -2151,21 +2163,53 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo >>>> * fails, we just zero-fill it. Live with it. >>>> */ >>>> if (unlikely(!src)) { >>>> - void *kaddr = kmap_atomic(dst); >>>> - void __user *uaddr = (void __user *)(va & PAGE_MASK); >>>> + void *kaddr; >>>> + pte_t entry; >>>> + void __user *uaddr = (void __user *)(addr & PAGE_MASK); >>>> >>>> + /* On architectures with software "accessed" bits, we would >>>> + * take a double page fault, so mark it accessed here. >>>> + */ > [...] >>>> + if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) { >>>> + vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, >>>> + &vmf->ptl); >>>> + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) { >>>> + entry = pte_mkyoung(vmf->orig_pte); >>>> + if (ptep_set_access_flags(vma, addr, >>>> + vmf->pte, entry, 0)) >>>> + update_mmu_cache(vma, addr, vmf->pte); >>>> + } else { >>>> + /* Other thread has already handled the fault >>>> + * and we don't need to do anything. If it's >>>> + * not the case, the fault will be triggered >>>> + * again on the same address. >>>> + */ >>>> + pte_unmap_unlock(vmf->pte, vmf->ptl); >>>> + return false; >>>> + } >>>> + pte_unmap_unlock(vmf->pte, vmf->ptl); >>>> + } > [...] >>>> + >>>> + kaddr = kmap_atomic(dst); >>> Since you moved the kmap_atomic() here, could the above >>> arch_faults_on_old_pte() run in a preemptible context? I suggested to >>> add a WARN_ON in patch 2 to be sure. >> Should I move kmap_atomic back to the original line? Thus, we can make sure >> that arch_faults_on_old_pte() is in the context of preempt_disabled? >> Otherwise, arch_faults_on_old_pte() may cause plenty of warning if I add >> a WARN_ON in arch_faults_on_old_pte. I tested it when I enable the PREEMPT=y >> on a ThunderX2 qemu guest. > So we have two options here: > > 1. Change arch_faults_on_old_pte() scope to the whole system rather than > just the current CPU. You'd have to wire up a new arm64 capability > for the access flag but this way we don't care whether it's > preemptible or not. > > 2. Keep the arch_faults_on_old_pte() per-CPU but make sure we are not > preempted here. The kmap_atomic() move would do but you'd have to > kunmap_atomic() before the return. > > I think the answer to my question below also has some implication on > which option to pick: > >>>> /* >>>> * This really shouldn't fail, because the page is there >>>> * in the page tables. But it might just be unreadable, >>>> * in which case we just give up and fill the result with >>>> * zeroes. >>>> */ >>>> - if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) >>>> + if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) { >>>> + /* Give a warn in case there can be some obscure >>>> + * use-case >>>> + */ >>>> + WARN_ON_ONCE(1); >>> That's more of a question for the mm guys: at this point we do the >>> copying with the ptl released; is there anything else that could have >>> made the pte old in the meantime? I think unuse_pte() is only called on >>> anonymous vmas, so it shouldn't be the case here. > If we need to hold the ptl here, you could as well have an enclosing > kmap/kunmap_atomic (option 2) with some goto instead of "return false". I am not 100% sure that I understand your suggestion well, so I drafted the patch here: Changes: optimize the indentions      hold the ptl longer -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma) +static inline bool cow_user_page(struct page *dst, struct page *src, +                 struct vm_fault *vmf)  { +    struct vm_area_struct *vma = vmf->vma; +    struct mm_struct *mm = vma->vm_mm; +    unsigned long addr = vmf->address; +    bool ret; +    pte_t entry; +    void *kaddr; +    void __user *uaddr; +      debug_dma_assert_idle(src); +    if (likely(src)) { +        copy_user_highpage(dst, src, addr, vma); +        return true; +    } +      /*       * If the source page was a PFN mapping, we don't have       * a "struct page" for it. We do a best-effort copy by       * just copying from the original user address. If that       * fails, we just zero-fill it. Live with it.       */ -    if (unlikely(!src)) { -        void *kaddr = kmap_atomic(dst); -        void __user *uaddr = (void __user *)(va & PAGE_MASK); +    kaddr = kmap_atomic(dst); +    uaddr = (void __user *)(addr & PAGE_MASK); + +    /* +     * On architectures with software "accessed" bits, we would +     * take a double page fault, so mark it accessed here. +     */ +    vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl); +    if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) { +        if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) { +            /* +             * Other thread has already handled the fault +             * and we don't need to do anything. If it's +             * not the case, the fault will be triggered +             * again on the same address. +             */ +            ret = false; +            goto pte_unlock; +        } + +        entry = pte_mkyoung(vmf->orig_pte); +        if (ptep_set_access_flags(vma, addr, vmf->pte, entry, 0)) +            update_mmu_cache(vma, addr, vmf->pte); +    } +    /* +     * This really shouldn't fail, because the page is there +     * in the page tables. But it might just be unreadable, +     * in which case we just give up and fill the result with +     * zeroes. +     */ +    if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {          /* -         * This really shouldn't fail, because the page is there -         * in the page tables. But it might just be unreadable, -         * in which case we just give up and fill the result with -         * zeroes. +         * Give a warn in case there can be some obscure +         * use-case           */ -        if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) -            clear_page(kaddr); -        kunmap_atomic(kaddr); -        flush_dcache_page(dst); -    } else -        copy_user_highpage(dst, src, va, vma); +        WARN_ON_ONCE(1); +        clear_page(kaddr); +    } + +    ret = true; + +pte_unlock: +    pte_unmap_unlock(vmf->pte, vmf->ptl); +    kunmap_atomic(kaddr); +    flush_dcache_page(dst); + +    return ret;  } --- Cheers, Justin (Jia He)