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[2003:d8:2f4c:df00:a9f5:b75b:33c:a17f]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4563b740e55sm93671305e9.17.2025.07.21.02.41.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 21 Jul 2025 02:41:19 -0700 (PDT) Message-ID: <6f16f99d-2f60-4e0d-a5c7-2dfdeb08bedd@redhat.com> Date: Mon, 21 Jul 2025 11:41:18 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] smaps: fix BUG_ON in smaps_hugetlb_range To: Michal Hocko Cc: Jinjiang Tu , akpm@linux-foundation.org, catalin.marinas@arm.com, lorenzo.stoakes@oracle.com, thiago.bauermann@linaro.org, superman.xpt@gmail.com, christophe.leroy@csgroup.eu, brahmajit.xyz@gmail.com, andrii@kernel.org, avagin@gmail.com, baolin.wang@linux.alibaba.com, ryan.roberts@arm.com, hughd@google.com, rientjes@google.com, joern@logfs.org, linux-mm@kvack.org, wangkefeng.wang@huawei.com References: <20250721081444.277183-1-tujinjiang@huawei.com> <46a22e7c-51f0-4fd0-8583-447c5e366029@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 3C68140003 X-Stat-Signature: z6axpu1zxdpmcuf7f7nqncsbec6rer1n X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1753090886-402846 X-HE-Meta: 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 s8BGuxHS 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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 21.07.25 11:35, Michal Hocko wrote: > On Mon 21-07-25 11:29:52, David Hildenbrand wrote: >> On 21.07.25 10:14, Jinjiang Tu wrote: >>> smaps_hugetlb_range() handles the pte without holdling ptl, and may be >>> concurrenct with migration, leaing to BUG_ON in pfn_swap_entry_to_page(). >>> The race is as follows. >>> >>> smaps_hugetlb_range migrate_pages >>> huge_ptep_get >>> remove_migration_ptes >>> folio_unlock >>> pfn_swap_entry_folio >>> BUG_ON >>> >>> To fix it, hold ptl lock in smaps_hugetlb_range(). >>> >>> Fixes: 25ee01a2fca0 ("mm: hugetlb: proc: add hugetlb-related fields to /proc/PID/smaps") >>> Signed-off-by: Jinjiang Tu >>> --- >>> fs/proc/task_mmu.c | 6 +++++- >>> 1 file changed, 5 insertions(+), 1 deletion(-) >>> >>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c >>> index 751479eb128f..0102ab3aaec1 100644 >>> --- a/fs/proc/task_mmu.c >>> +++ b/fs/proc/task_mmu.c >>> @@ -1020,10 +1020,13 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask, >>> { >>> struct mem_size_stats *mss = walk->private; >>> struct vm_area_struct *vma = walk->vma; >>> - pte_t ptent = huge_ptep_get(walk->mm, addr, pte); >>> struct folio *folio = NULL; >>> bool present = false; >>> + spinlock_t *ptl; >>> + pte_t ptent; >>> + ptl = huge_pte_lock(hstate_vma(vma), walk->mm, pte); >>> + ptent = huge_ptep_get(walk->mm, addr, pte); >>> if (pte_present(ptent)) { >>> folio = page_folio(pte_page(ptent)); >>> present = true; >>> @@ -1042,6 +1045,7 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask, >>> else >>> mss->private_hugetlb += huge_page_size(hstate_vma(vma)); >>> } >>> + spin_unlock(ptl); >>> return 0; >>> } >>> #else >> >> >> Heh, I stumbled over that code many times and wondered "why don't we need >> the PTL here -- I'm sure it's fine because otherwise we would be getting >> reports.". >> >> In pagewalk code we only hold the vma lock -- see walk_hugetlb_range(). >> >> So I think we should just grab the PTL in all these walkers. > > I believe the reason that we try to avoid taking the lock in these paths > is that they are userspace accessible and we do not want to expose them > to users. I think it would be good to try to rework the code to not > require the lock even if we get imprecise numbers. We cannot trigger any > oops of course and that is a clear bug here. Can we achieve the fix > without taking the lock? We grab PTLs whenever we walk page tables, except in hugetlb. So I much rather want that changed? -- Cheers, David / dhildenb