The patch titled Subject: mm/khugepaged: fix collapse_pte_mapped_thp() to allow anon_vma has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-khugepaged-fix-collapse_pte_mapped_thp-to-allow-anon_vma.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------ From: Hugh Dickins hughd@google.com Subject: mm/khugepaged: fix collapse_pte_mapped_thp() to allow anon_vma Date: Thu, 22 Dec 2022 12:41:50 -0800 (PST)
uprobe_write_opcode() uses collapse_pte_mapped_thp() to restore huge pmd, when removing a breakpoint from hugepage text: vma->anon_vma is always set in that case, so undo the prohibition. And MADV_COLLAPSE ought to be able to collapse some page tables in a vma which happens to have anon_vma set from CoWing elsewhere.
Is anon_vma lock required? Almost not: if any page other than expected subpage of the non-anon huge page is found in the page table, collapse is aborted without making any change. However, it is possible that an anon page was CoWed from this extent in another mm or vma, in which case a concurrent lookup might look here: so keep it away while clearing pmd (but perhaps we shall go back to using pmd_lock() there in future).
Note that collapse_pte_mapped_thp() is exceptional in freeing a page table without having cleared its ptes: I'm uneasy about that, and had thought pte_clear()ing appropriate; but exclusive i_mmap lock does fix the problem, and we would have to move the mmu_notification if clearing those ptes.
What this fixes is not a dangerous instability. But I suggest Cc stable because uprobes "healing" has regressed in that way, so this should follow 8d3c106e19e8 into those stable releases where it was backported (and may want adjustment there - I'll supply backports as needed).
Link: https://lkml.kernel.org/r/b740c9fb-edba-92ba-59fb-7a5592e5dfc@google.com Fixes: 8d3c106e19e8 ("mm/khugepaged: take the right locks for page table retraction") Signed-off-by: Hugh Dickins hughd@google.com Cc: Jann Horn jannh@google.com Cc: Yang Shi shy828301@gmail.com Cc: David Hildenbrand david@redhat.com Cc: Zach O'Keefe zokeefe@google.com Cc: Song Liu songliubraving@fb.com Cc: stable@vger.kernel.org [5.4+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
--- a/mm/khugepaged.c~mm-khugepaged-fix-collapse_pte_mapped_thp-to-allow-anon_vma +++ a/mm/khugepaged.c @@ -1460,14 +1460,6 @@ int collapse_pte_mapped_thp(struct mm_st if (!hugepage_vma_check(vma, vma->vm_flags, false, false, false)) return SCAN_VMA_CHECK;
- /* - * Symmetry with retract_page_tables(): Exclude MAP_PRIVATE mappings - * that got written to. Without this, we'd have to also lock the - * anon_vma if one exists. - */ - if (vma->anon_vma) - return SCAN_VMA_CHECK; - /* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */ if (userfaultfd_wp(vma)) return SCAN_PTE_UFFD_WP; @@ -1567,8 +1559,14 @@ int collapse_pte_mapped_thp(struct mm_st }
/* step 4: remove pte entries */ + /* we make no change to anon, but protect concurrent anon page lookup */ + if (vma->anon_vma) + anon_vma_lock_write(vma->anon_vma); + collapse_and_free_pmd(mm, vma, haddr, pmd);
+ if (vma->anon_vma) + anon_vma_unlock_write(vma->anon_vma); i_mmap_unlock_write(vma->vm_file->f_mapping);
maybe_install_pmd: _
Patches currently in -mm which might be from hughd@google.com are
mm-khugepaged-fix-collapse_pte_mapped_thp-to-allow-anon_vma.patch