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[217.140.110.172]) by mx.google.com with ESMTP id f6si18745589eja.338.2019.07.31.23.40.44 for ; Wed, 31 Jul 2019 23:40:44 -0700 (PDT) Received-SPF: pass (google.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) client-ip=217.140.110.172; Authentication-Results: mx.google.com; spf=pass (google.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=anshuman.khandual@arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id AF3BA337; Wed, 31 Jul 2019 23:40:43 -0700 (PDT) Received: from [10.163.1.81] (unknown [10.163.1.81]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 84E463F694; Wed, 31 Jul 2019 23:42:33 -0700 (PDT) Subject: Re: [PATCH v9 12/21] mm: pagewalk: Allow walking without vma To: Steven Price , linux-mm@kvack.org Cc: Mark Rutland , x86@kernel.org, Arnd Bergmann , Ard Biesheuvel , Peter Zijlstra , Catalin Marinas , Dave Hansen , linux-kernel@vger.kernel.org, =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Ingo Molnar , Borislav Petkov , Andy Lutomirski , "H. Peter Anvin" , James Morse , Thomas Gleixner , Will Deacon , Andrew Morton , linux-arm-kernel@lists.infradead.org, "Liang, Kan" References: <20190722154210.42799-1-steven.price@arm.com> <20190722154210.42799-13-steven.price@arm.com> <7fc50563-7d5d-7270-5a6a-63769e9c335a@arm.com> <5aff70f7-67a5-c7e8-5fec-8182dea0da0c@arm.com> From: Anshuman Khandual Message-ID: Date: Thu, 1 Aug 2019 12:11:05 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <5aff70f7-67a5-c7e8-5fec-8182dea0da0c@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US 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 07/29/2019 05:59 PM, Steven Price wrote: > On 28/07/2019 15:20, Anshuman Khandual wrote: >> >> >> On 07/22/2019 09:12 PM, Steven Price wrote: >>> Since 48684a65b4e3: "mm: pagewalk: fix misbehavior of walk_page_range >>> for vma(VM_PFNMAP)", page_table_walk() will report any kernel area as >>> a hole, because it lacks a vma. >>> >>> This means each arch has re-implemented page table walking when needed, >>> for example in the per-arch ptdump walker. >>> >>> Remove the requirement to have a vma except when trying to split huge >>> pages. >>> >>> Signed-off-by: Steven Price >>> --- >>> mm/pagewalk.c | 25 +++++++++++++++++-------- >>> 1 file changed, 17 insertions(+), 8 deletions(-) >>> >>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c >>> index 98373a9f88b8..1cbef99e9258 100644 >>> --- a/mm/pagewalk.c >>> +++ b/mm/pagewalk.c >>> @@ -36,7 +36,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end, >>> do { >>> again: >>> next = pmd_addr_end(addr, end); >>> - if (pmd_none(*pmd) || !walk->vma) { >>> + if (pmd_none(*pmd)) { >>> if (walk->pte_hole) >>> err = walk->pte_hole(addr, next, walk); >>> if (err) >>> @@ -59,9 +59,14 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end, >>> if (!walk->pte_entry) >>> continue; >>> >>> - split_huge_pmd(walk->vma, pmd, addr); >>> - if (pmd_trans_unstable(pmd)) >>> - goto again; >>> + if (walk->vma) { >>> + split_huge_pmd(walk->vma, pmd, addr); >> >> Check for a PMD THP entry before attempting to split it ? > > split_huge_pmd does the check for us: >> #define split_huge_pmd(__vma, __pmd, __address) \ >> do { \ >> pmd_t *____pmd = (__pmd); \ >> if (is_swap_pmd(*____pmd) || pmd_trans_huge(*____pmd) \ >> || pmd_devmap(*____pmd)) \ >> __split_huge_pmd(__vma, __pmd, __address, \ >> false, NULL); \ >> } while (0) > > And this isn't a change from the previous code - only that the entry is > no longer split when walk->vma==NULL. Does it make sense to name walk->vma check to differentiate between user and kernel page tables. IMHO that will help make things clear and explicit during page table walk. > >>> + if (pmd_trans_unstable(pmd)) >>> + goto again; >>> + } else if (pmd_leaf(*pmd)) { >>> + continue; >>> + } >>> + >>> err = walk_pte_range(pmd, addr, next, walk); >>> if (err) >>> break; >>> @@ -81,7 +86,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end, >>> do { >>> again: >>> next = pud_addr_end(addr, end); >>> - if (pud_none(*pud) || !walk->vma) { >>> + if (pud_none(*pud)) { >>> if (walk->pte_hole) >>> err = walk->pte_hole(addr, next, walk); >>> if (err) >>> @@ -95,9 +100,13 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end, >>> break; >>> } >>> >>> - split_huge_pud(walk->vma, pud, addr); >>> - if (pud_none(*pud)) >>> - goto again; >>> + if (walk->vma) { >>> + split_huge_pud(walk->vma, pud, addr); >> >> Check for a PUD THP entry before attempting to split it ? > > Same as above. > >>> + if (pud_none(*pud)) >>> + goto again; >>> + } else if (pud_leaf(*pud)) { >>> + continue; >>> + } >> >> This is bit cryptic. walk->vma check should be inside a helper is_user_page_table() >> or similar to make things clear. p4d_leaf() check missing in walk_p4d_range() for >> kernel page table walk ? Wondering if p?d_leaf() test should be moved earlier while >> calling p?d_entry() for kernel page table walk. > > I wasn't sure if it was worth putting p4d_leaf() and pgd_leaf() checks > in (yet). No architecture that I know of uses such large pages. Just to be complete it does make sense to add the remaining possible leaf entry checks but will leave it upto you. > > I'm not sure what you mean by moving the p?d_leaf() test earlier? Can > you explain with an example? In case its a kernel p?d_leaf() entry, then there is nothing to be done after calling respective walk->p?d_entry() functions. Hence this check should not complement user page table check (walk->vma) later in the function but instead be checked right after walk->p?d_entry(). But its not a big deal I guess.