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Shutemov" To: Catalin Marinas Subject: Re: [PATCH v8 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared Message-ID: <20190924115913.ju67nr4gcdbzbeva@box> 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> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20190924103324.GB41214@arrakis.emea.arm.com> User-Agent: NeoMutt/20180716 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190924_045914_765511_AC546EBC X-CRM114-Status: GOOD ( 29.10 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , "linux-mm@kvack.org" , Punit Agrawal , Will Deacon , Alex Van Brunt , "Justin He \(Arm Technology China\)" , Marc Zyngier , Anshuman Khandual , Matthew Wilcox , "Kaly Xin \(Arm Technology China\)" , "hejianet@gmail.com" , Ralph Campbell , Suzuki Poulose , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Thomas Gleixner , nd , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , James Morse , Andrew Morton , Robin Murphy , "Kirill A. Shutemov" Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Sep 24, 2019 at 11:33:25AM +0100, 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". Yeah, look like we need to hold ptl for longer. There is nothing I see that would prevent clearing young bit under us otherwise. -- Kirill A. Shutemov _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel