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From: Lance Yang In-Reply-To: <6b11bf0a-769c-4ef2-ac6f-2af38200a6bc@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: ifqdiig1sww1udwdbim3ukrefdi95ref X-Rspamd-Queue-Id: DB0C620002 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1780310901-295199 X-HE-Meta: U2FsdGVkX1+7Qg7odMi38O3XTmYJaslP7DF8+mOrwwltPebgl4YkgtOi8ZBH9ljg2I7IYvSf1icsRlno+QAwynDF5qpdYhMc1PCoofY5sW6y9yEnErpDHE8V8AnknXGXs/GOSNax/9EcbjrqNf9ZjcPsXPgzOujaP5lawRmTifsVLXOB+Sx6NuiWvpi0uEBIpIBJv/Hug+p+fBOcBw85rUDYpcdvkb71u8riyRiuxzuiCfRUcZYjsHzkKw3zYfSQzJBnLmCSFoYe5i1QOq8NGX2+Fcxc6eofsKiT7FixwAUKIs5/2Q+iaEw3RvQmTsMMYZw9+lfzxPLgDvfxk1NW9zgArCOjAJBKj6pzFqeS+2WdfitI2dKQZrc2OxmIHL6M+8idM282ZRKSYDQFY6aWEDgpgmhTEvd8wCBiQxamGdyejbAXBOjazXjoQa3I+f2aZo+vFGZ68BSKC9j2sBG6VxjPveZVYEHgnhB9Ku1lbuSeqVgbWpAD2Ffp5NmBXSp6rEHDRNwN5X6xVVbM2TOnMoDqrK/rXIK5RtDhJ8wmRlP8IQuAOjHeToh7bPU2wVn+98btQ1mQ5PL2tKbpXL072StvD6j6MP828NoXJJZoQoU2mZ7oe5+4TJ+d5CauDoh7AzH3+F+r4/hpmO6V+UEocFtpT7LTEObpvateRR4emD5mby13LKhWov60Pce7XDp+6owoA7mZ/dhZssYxTqWLC7F+4sma3xoaLyKvVQqGbjCdfGTNlcxN60r4DmF+RR6aKmm1vaRRcdw+PkSGGrXBnJc7rlFuP/N4JAB503twCrReYlon2ho/lx7opVbhBK4TCegKY2Xtdx4T3i5hmEwV3E/ejIU9zembsyPxEHH4nzaBdIzMJ805y9guTF6n9LPnzkOrqsFtcSCz28Fhq2ve3+9G56buczO2yT2+cR8z/8UunY89d59Y7KQeFOllBtZhNZLwlVjhST+veBJ97jj 2/Ev7B5b FWumortAJ5t1YoTizFqbTy6Rsvy6mhH90EydY87L6iRazcMtMUKtY4EeRqGEUl/ZkaB03SHpQhlteZ3ldifPYI9jLggUXEZ9HrrtiBFnP4y38UKaRDJD6vFqA82knMj8UphT/1OgqiTpt11CHcvPV4sXr9W6gtZLVLEn8SN385RDblZUebDkSq+BI/1ufHHd+ne8O/9896n/HpI0iZCF9/YR5446ju8ATkYUPiRpdVedn1ewU2x/yRRg2Xzr7XwzuUP+B3pJU0mDL/biY2nKjDs26YEF9Yz/VwxiOk9PH8xl4AqBw2xbLPmV3MhFD1WzQeSmepXPEQeaZ3fOFsqCb0TZjjA9UGrQNsq9OMKC3JQzZDPjjP2v7vPr6FY8nCrMoJd9X Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026/6/1 18:23, David Hildenbrand (Arm) wrote: > 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? Ah, cool! __folio_mark_uptodate() already does the job :P So yeah, no extra smp_wmb() needed here! Cheers, Lance