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AJvYcCV2whYo8iS+/cYYDQYIzk5Q77gT0piM+BcP6H039Wud6AJaQCCi8gGUw9Lu4C8nKVd3NiZwwI3QrmR+XARdZCVSwek= X-Gm-Message-State: AOJu0YwSI8v5cOIdOv1qo7ke7jEaMqBU1AmHmaUc5R94paysTMwn2uEA TTCIlqV0KzvlY/G/f4XIOgxty5f+JGl/fX4HZch7mwgQoIW7uDdN X-Google-Smtp-Source: AGHT+IEF/J0DHWSNcDOcZmL/hIiqI/9PDuGnKLhV3Qcp0yU1M5alvi1nAMXSzoSut1e/uHxnGCJxBg== X-Received: by 2002:a05:6a00:2284:b0:6e7:821b:e972 with SMTP id f4-20020a056a00228400b006e7821be972mr1811564pfe.29.1710936194212; Wed, 20 Mar 2024 05:03:14 -0700 (PDT) Received: from [192.168.255.10] ([43.132.141.21]) by smtp.gmail.com with ESMTPSA id h25-20020a62b419000000b006e739e09504sm3925090pfn.137.2024.03.20.05.03.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 Mar 2024 05:03:13 -0700 (PDT) Message-ID: Date: Wed, 20 Mar 2024 20:03:10 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 01/12] mm/ksm: catch tail page abnormal in page_stable_node Content-Language: en-US To: David Hildenbrand , alexs@kernel.org, kasong@tencent.com, Andrew Morton , "open list:MEMORY MANAGEMENT" , open list References: <20240318121443.3991104-1-alexs@kernel.org> <7a0d7d0e-d241-4942-b6c1-e60d597b3dd6@redhat.com> <54a8ec22-ce00-425c-bb58-54cf8606a362@gmail.com> <280409c6-3479-4c6b-a94e-1d90994917cd@redhat.com> From: Alex Shi In-Reply-To: <280409c6-3479-4c6b-a94e-1d90994917cd@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 8A40A1C0011 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: aakouk944qa3f6prrtutx7uz6xsmn6zy X-HE-Tag: 1710936195-198861 X-HE-Meta: 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 0tszqJDW 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 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: List-Subscribe: List-Unsubscribe: On 3/20/24 5:29 PM, David Hildenbrand wrote: > On 20.03.24 10:05, Alex Shi wrote: >> >> >> On 3/18/24 8:25 PM, David Hildenbrand wrote: >>> On 18.03.24 13:14, alexs@kernel.org wrote: >>>> From: Alex Shi >>>> >>>> commit 19138349ed59 ("mm/migrate: Add folio_migrate_flags()") change the >>>> meaning of func page_stable_node() to check the compound head for tail >>>> 'page' instead of tail page self. >>>> But seems both semantics are same at results, the func always return NULL >>>>    for tail page. So adding a bug monitor here in case of abnormal. >>>> >>>> Signed-off-by: Alex Shi >>>> Cc: Izik Eidus >>>> Cc: Matthew Wilcox >>>> Cc: Andrea Arcangeli >>>> Cc: Hugh Dickins >>>> Cc: Chris Wright >>>> To: linux-kernel@vger.kernel.org >>>> To: linux-mm@kvack.org >>>> To: Andrew Morton >>>> --- >>>>    mm/ksm.c | 7 ++++++- >>>>    1 file changed, 6 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/mm/ksm.c b/mm/ksm.c >>>> index 8c001819cf10..3ff469961927 100644 >>>> --- a/mm/ksm.c >>>> +++ b/mm/ksm.c >>>> @@ -1091,7 +1091,12 @@ static inline struct ksm_stable_node *folio_stable_node(struct folio *folio) >>>>      static inline struct ksm_stable_node *page_stable_node(struct page *page) >>>>    { >>>> -    return folio_stable_node(page_folio(page)); >>>> +    struct ksm_stable_node *node; >>>> + >>>> +    node = folio_stable_node(page_folio(page)); >>>> +    VM_BUG_ON_PAGE(PageTail(page) && node, page); >>> >>> I don't really understand why we would want this. >>> >>> Only KSM folios can have a node in the stable tree. KSM folios cannot be large folios. At that is precisely what folio_stable_node() checks. >>> >>> If we'd have a large folio identify as a KSM folio we'd be in much bigger trouble. >>> >>> >>> Besides, I'm sure you read "22) Do not crash the kernel" in Documentation/process/coding-style.rst >>> >> >> Hi David, >> >> Thanks for comments! >> Forgive my stupidity, I understand KSM stable tree has no compound pages, but when searching a tail page in ksm_do_scan(), why we couldn't be in a race, that another VM doing THP collapse on the same contents pages, while the 3rd vm is doing hugepage spliting? > > We always call cmp_and_merge_page() while holding a reference on the page. > > There, we call page_stable_node() directly and via stable_tree_search()->page_stable_node() on that page. > > When stable_tree_search() returns a kpage, we also hold a reference to that kpage. So calling page_stable_node() on the kpage behaves the same. > > As we are holding page references, pages cannot be split/merged and we should not see any races in page_stable_node(). > > Am I missing something? > > Note that your change would also not help here: if it would be racy, you'd also not reliably catch any tail pages. > > But it should not be racy unless I am missing something. > Hi David, Thanks for the info, I see. BTW, I should cc you for my KSM folio patchset review: https://lore.kernel.org/linux-mm/20240320074049.4130552-2-alexs@kernel.org/ Best regards! Alex