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b=c833BeYwgWz0ppp2x5phqCnJSunVgereyjGFUP4VqomH4E1+QpFfXKobIPxqHWZjf mHsIUimognUz54K96CBjCRux7u5M+O3Fns9wbdjp0/SfXsM5HOfeyQIKeIBQ2+IxQK VdGUz2DB8VatOJWgXIaufLTsE/5ra+O+5teh72LQe+jWlTOSUmxZtZsSojv2M9Jifd kicJVxsrzwL2zbz0aLVT4FgrsgN2rKBfx/N+CsNTRqdmNCn6X9BBnbueQgumucs+I+ eHf+Qiz0MgWpWxRaJbOr7V12LUXNhYpe4ePGGEX9K2h7Mg7rr/KZ+q6an5DZThkQhv gqc+JIOA7PrpA== Message-ID: <6b11bf0a-769c-4ef2-ac6f-2af38200a6bc@kernel.org> Date: Mon, 1 Jun 2026 12:23:33 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v18 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse To: Lance Yang Cc: npache@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, liam@infradead.org, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com, usama.arif@linux.dev References: <2024af56-5e99-4799-a586-e9ba756cecb9@kernel.org> <20260601032804.96122-1-lance.yang@linux.dev> <616de1a8-1cfd-40b8-b04f-7b324be40bfd@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 98C904000E X-Stat-Signature: z7n9y3c4uhbdrbqwf8tfayhwf4iobqzq X-HE-Tag: 1780309431-630231 X-HE-Meta: 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 Ondk5uT3 Vl6BTlhRD2SiBS2Sv64/35ntCsmBOjVvj8UcLME+Ddi0pVx1WD0bWxNG2GlJLLSpwvfngVdYkeA3GBPNLZ5Y35KjcBh8oHSx00NegAftTvnAd07GxWuiwhUB3tNCmJRx2ut7l9SmeZ06zS0ybQJcX+Q1HRGLA6PHE0yfp4NBelpd4NhRE/VT49Jxoxh4hswCI20DRg78+W/2Wvqu3S8iPaKG7VzOyRMQvJntpCSGFmqbNlJ2WUQgUwX5em/yw6/Ba4myqd6YNLRRAKpK8IMN7FqLuMOJHO+pvADRqs6X/JklHyTg3jiOKrK7x2xWcEy1Jum5UXlF1IsUBuIRaUvcTU9oz5SVQpARlB+TA5S9ZWFwsGdbL7qpcSCQ0C/wf6o0GZ8XFcxEHSwOudNaeswoZ96k9zw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/1/26 11:08, Lance Yang wrote: > > > On 2026/6/1 14:54, David Hildenbrand (Arm) wrote: >> On 6/1/26 05:28, Lance Yang wrote: >>> >>> >>> Ah, fair point. >>> >>> I was mostly worried about arch hooks that walk vma->vm_mm again, rather >>> than only using the pte pointer passed in. For example, mips does: >> >> Right, a re-walk would be the real problem. >> >>> >>>    update_mmu_cache_range() >>>      -> __update_tlb() >>>        -> pgd_offset(vma->vm_mm, address) >>>        -> pte_offset_map(...) >>> >>> and __update_tlb() has this assumption: >>> >>>         /* >>>          * update_mmu_cache() is called between pte_offset_map_lock() >>>          * and pte_unmap_unlock(), so we can assume that ptep is not >>>          * NULL here: and what should be done below if it were NULL? >>>          */ >>> >>> So if khugepaged happens to run with current->active_mm == vma->vm_mm >>> here, could __update_tlb() hit the none PMD, get NULL from >>> pte_offset_map(), and then dereference it? >> >> Likely yes -- that MIPS code is horrible. And the comment in MIPS code >> even spells that out. :( >> >> Do you know about other code like that, or is MIPS the only one doing a >> re-walk and crossing fingers? >> >>> >>> Just wanted to raise it since some arch code may still have assumptions >>> like this, and the always-enable-mTHP work is getting closer ... >> >> Right. I assume set_pte_at() couldn't trigger something similar (re-walk) in >> arch code, >> because we simply provide the ptep. update_mmu_cache_range() only consumes the >> pte. >> >>> >>> Probably very very very hard to hit, though :) >> >> Delaying update_mmu_cache_range() is nasty, as we'd have to make sure that >> nobody can interfere in the meantime ... and the PMD lock will not be sufficient. >> >> Maybe we could reinstall the page table with the cleared (none) entries while >> still holding the PTL? >> >> Thinking out loud: >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 5ba298d420b7..e39b750b1e6f 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -1413,13 +1413,17 @@ static enum scan_result collapse_huge_page(struct >> mm_struct *mm, unsigned long s >>                  map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr); >>          } else { >>                  /* >> -                * set_ptes is called in map_anon_folio_pte_nopf with the >> -                * pmd_ptl lock still held; this is safe as the PMD is expected >> -                * to be none. The pmd entry is then repopulated below. >> +                * Re-insert the page table with the cleared entries, but >> +                * hold the PTL, such that no one can mess with the re-installed >> +                * page table until we updated the temporarily-cleared entries >> +                * through map_anon_folio_pte_nopf(). >>                   */ >> -               map_anon_folio_pte_nopf(folio, pte, vma, start_addr, / >> *uffd_wp=*/ false); >> -               smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */ > > One small thing, I think we should probably keep the smp_wmb(), and just > move it before the earlier pmd_populate(). > > IIUC, the ordering we want is still: > >   clear old PTEs >   smp_wmb() >   pmd_populate() > > so another CPU cannot walk through the re-installed PMD and still observe > the old PTEs, right? There is a smp_wmb() in __folio_mark_uptodate(), that should be sufficient? -- Cheers, David