On Tue, Jun 02, 2026 at 09:30:06AM -0600, Nico Pache wrote: >On Mon, Jun 1, 2026 at 4:48 AM Lance Yang <[email protected]> wrote: >> >> >> >> 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! > >are we sure? that folio_mark_uptodate is done before the PTEs are >reinstalled. Then we reinstall the PMD right after. Currently >separated by the smp_wmb().
Reinstalling the PMD first makes the PTE table reachable again, right? So before pmd_populate(), we only need to order the old PTE clears before the PTE table is reachable again; __folio_mark_uptodate() already has the smp_wmb() for that :) The new PTEs are filled later under the PTL. Hopefully I didn't miss soemthing :) >I was copying this from other THP code that performs similar PTE/PMD juggling. > >I can remove it, but I'd rather air on the side of caution with this. Cheers, Lance
