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Shutemov" To: Catalin Marinas Cc: Jia He , 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 , Jun Yao , Alex Van Brunt , Robin Murphy , Thomas Gleixner , Andrew Morton , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Ralph Campbell , hejianet@gmail.com, Kaly Xin Subject: Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared Message-ID: <20190919150007.k7scjplcya53j7r4@box> References: <20190918131914.38081-1-justin.he@arm.com> <20190918131914.38081-4-justin.he@arm.com> <20190918140027.ckj32xnryyyesc23@box> <20190918180029.GB20601@iMac.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190918180029.GB20601@iMac.local> User-Agent: NeoMutt/20180716 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 18, 2019 at 07:00:30PM +0100, Catalin Marinas wrote: > On Wed, Sep 18, 2019 at 05:00:27PM +0300, Kirill A. Shutemov wrote: > > On Wed, Sep 18, 2019 at 09:19:14PM +0800, Jia He wrote: > > > @@ -2152,20 +2163,34 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo > > > */ > > > if (unlikely(!src)) { > > > void *kaddr = kmap_atomic(dst); > > > - void __user *uaddr = (void __user *)(va & PAGE_MASK); > > > + void __user *uaddr = (void __user *)(addr & PAGE_MASK); > > > + pte_t entry; > > > > > > /* > > > * 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. > > > + * zeroes. On architectures with software "accessed" bits, > > > + * we would take a double page fault here, so mark it > > > + * accessed here. > > > */ > > > + if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) { > > > + spin_lock(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); > > > + } > > > > I don't follow. > > > > So if pte has changed under you, you don't set the accessed bit, but never > > the less copy from the user. > > > > What makes you think it will not trigger the same problem? > > > > I think we need to make cow_user_page() fail in this case and caller -- > > wp_page_copy() -- return zero. If the fault was solved by other thread, we > > are fine. If not userspace would re-fault on the same address and we will > > handle the fault from the second attempt. > > It would be nice to clarify the semantics of this function and do as > you suggest but the current comment is slightly confusing: > > /* > * 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. > */ > > Would any user-space rely on getting a zero-filled page here instead of > a recursive fault? I don't see the point in zero-filled page in this case. SIGBUS sounds like more appropriate response, no? -- Kirill A. Shutemov