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[104.129.187.94]) by smtp.gmail.com with ESMTPSA id d76sm7941194pfd.185.2019.09.21.06.19.38 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 21 Sep 2019 06:19:52 -0700 (PDT) Subject: Re: [PATCH v7 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared To: Matthew Wilcox , Jia He Cc: Catalin Marinas , Will Deacon , Mark Rutland , James Morse , Marc Zyngier , "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 , nd@arm.com References: <20190920135437.25622-1-justin.he@arm.com> <20190920135437.25622-4-justin.he@arm.com> <20190920155300.GC15392@bombadil.infradead.org> From: Jia He Message-ID: Date: Sat, 21 Sep 2019 21:19:34 +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: <20190920155300.GC15392@bombadil.infradead.org> Content-Type: text/plain; charset=gbk; format=flowed Content-Transfer-Encoding: 7bit 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 behalf of justin.he@arm.com] Hi Matthew On 2019/9/20 23:53, Matthew Wilcox wrote: > On Fri, Sep 20, 2019 at 09:54:37PM +0800, Jia He wrote: >> -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma) >> +static inline int cow_user_page(struct page *dst, struct page *src, >> + struct vm_fault *vmf) >> { > Can we talk about the return type here? > >> + } 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 -1; > ... >> + return 0; >> } > So -1 for "try again" and 0 for "succeeded". > >> + if (cow_user_page(new_page, old_page, vmf)) { > Then we use it like a bool. But it's kind of backwards from a bool because > false is success. > >> + /* COW failed, if the fault was solved by other, >> + * it's fine. If not, userspace would re-fault on >> + * the same address and we will handle the fault >> + * from the second attempt. >> + */ >> + put_page(new_page); >> + if (old_page) >> + put_page(old_page); >> + return 0; > And we don't use the return value; in fact we invert it. > > Would this make more sense: > > static inline bool cow_user_page(struct page *dst, struct page *src, > struct vm_fault *vmf) > ... > pte_unmap_unlock(vmf->pte, vmf->ptl); > return false; > ... > return true; > ... > if (!cow_user_page(new_page, old_page, vmf)) { > > That reads more sensibly for me. We could also go with returning a > vm_fault_t, but that would be more complex than needed today, I think. Ok, will change the return type to bool as you suggested. Thanks --- Cheers, Justin (Jia He)