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Wed, 12 Aug 2020 09:22:54 +0000 (GMT) Received: from [9.85.71.17] (unknown [9.85.71.17]) by d06av22.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 12 Aug 2020 09:22:54 +0000 (GMT) Subject: Re: [PATCH 03/16] debug_vm_pgtable/set_pte: Don't use set_pte_at to update an existing pte entry To: Anshuman Khandual , linux-mm@kvack.org, akpm@linux-foundation.org Cc: mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org References: <20200812063358.369514-1-aneesh.kumar@linux.ibm.com> <20200812063358.369514-3-aneesh.kumar@linux.ibm.com> <94ed519a-2cf2-af50-82be-2a559dee7d86@arm.com> From: "Aneesh Kumar K.V" Message-ID: <32200d05-2707-694a-70b2-e93e5ae83b4a@linux.ibm.com> Date: Wed, 12 Aug 2020 14:52:54 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <94ed519a-2cf2-af50-82be-2a559dee7d86@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235,18.0.687 definitions=2020-08-12_02:2020-08-11,2020-08-12 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 spamscore=0 malwarescore=0 adultscore=0 mlxscore=0 bulkscore=0 clxscore=1015 lowpriorityscore=0 mlxlogscore=801 impostorscore=0 phishscore=0 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2008120064 X-Rspamd-Queue-Id: 4945D1814B0C8 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 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 8/12/20 2:42 PM, Anshuman Khandual wrote: > > > On 08/12/2020 12:03 PM, Aneesh Kumar K.V wrote: >> set_pte_at() should not be used to set a pte entry at locations that >> already holds a valid pte entry. Architectures like ppc64 don't do TLB >> invalidate in set_pte_at() and hence expect it to be used to set locations >> that are not a valid PTE. > > Even though set_pte_at() is not really a arch page table helper and > very much arch specific, I just wonder why this deviation on ppc64 > as compared to other platforms. Detecting such semantics variation > across platforms is an objective of this test. Not sure what you mean by set_pte_at is not a page table helper. Generic kernel use that helper to set a pte entry. Now w.r.t ppc64 behavior this was discussed multiple times. I guess Linux kernel always used set_pte_at on a none pte entry. We had some exceptions in the recent past. But all fixed when noticed. 383321ab8578dfe3bbcc0bc5604c0f8ae08a5c98 mm/hugetlb/migration: use set_huge_pte_at instead of set_pte_at cee216a696b2004017a5ecb583366093d90b1568 mm/autonuma: don't use set_pte_at when updating protnone ptes 56eecdb912b536a4fa97fb5bfe5a940a54d79be6 mm: Use ptep/pmdp_set_numa() for updating _PAGE_NUMA bit Yes. Having a testcase like this help > > As small nit. > > Please follow the existing subject format for all patches in here. > It will improve readability and also help understand these changes > better, later on. > > mm/debug_vm_pgtable: > >> >> Signed-off-by: Aneesh Kumar K.V >> --- >> mm/debug_vm_pgtable.c | 8 ++++---- >> 1 file changed, 4 insertions(+), 4 deletions(-) >> >> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c >> index 4c32063a8acf..02a7c20aa4a2 100644 >> --- a/mm/debug_vm_pgtable.c >> +++ b/mm/debug_vm_pgtable.c >> @@ -81,8 +81,6 @@ static void __init pte_advanced_tests(struct mm_struct *mm, >> pte = ptep_get(ptep); >> WARN_ON(pte_write(pte)); >> >> - pte = pfn_pte(pfn, prot); >> - set_pte_at(mm, vaddr, ptep, pte); >> ptep_get_and_clear(mm, vaddr, ptep); >> pte = ptep_get(ptep); >> WARN_ON(!pte_none(pte)); > > This makes sense. But could you please fold this code stanza with > the previous one in order to imply that 'ptep' did have some valid > entry before being cleared and checked against pte_none(). > will do that >> @@ -97,12 +95,14 @@ static void __init pte_advanced_tests(struct mm_struct *mm, >> pte = ptep_get(ptep); >> WARN_ON(!(pte_write(pte) && pte_dirty(pte))); >> >> - pte = pfn_pte(pfn, prot); >> - set_pte_at(mm, vaddr, ptep, pte); >> ptep_get_and_clear_full(mm, vaddr, ptep, 1); >> pte = ptep_get(ptep); >> WARN_ON(!pte_none(pte)); > > Same, please fold back. > ok >> + /* >> + * We should clear pte before we do set_pte_at >> + */ >> + pte = ptep_get_and_clear(mm, vaddr, ptep); >> pte = pte_mkyoung(pte); >> set_pte_at(mm, vaddr, ptep, pte); >> ptep_test_and_clear_young(vma, vaddr, ptep); >> > > The comment above should also explain details that are mentioned > in the commit message i.e how platforms such as ppc64 expects a > clear pte entry for set_pte_at() to work. > I don't think it is specific to ppc64. There is nothing specific to ppc64 architecture in there. It is an optimization used in kernel to help architecture avoid TLB flush. I will update the comment as below /* We should clear pte before we do set_pte_at so that set_pte_at don't find a valid pte at ptep *? is that good? -aneesh