This is a note to let you know that I've just added the patch titled khugepaged: collapse_pte_mapped_thp() protect the pmd lock to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: khugepaged-collapse_pte_mapped_thp-protect-the-pmd-lock.patch and it can be found in the queue-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 119a5fc16105b2b9383a6e2a7800b2ef861b2975 Mon Sep 17 00:00:00 2001 From: Hugh Dickins <hughd@xxxxxxxxxx> Date: Thu, 6 Aug 2020 23:26:18 -0700 Subject: khugepaged: collapse_pte_mapped_thp() protect the pmd lock From: Hugh Dickins <hughd@xxxxxxxxxx> commit 119a5fc16105b2b9383a6e2a7800b2ef861b2975 upstream. When retract_page_tables() removes a page table to make way for a huge pmd, it holds huge page lock, i_mmap_lock_write, mmap_write_trylock and pmd lock; but when collapse_pte_mapped_thp() does the same (to handle the case when the original mmap_write_trylock had failed), only mmap_write_trylock and pmd lock are held. That's not enough. One machine has twice crashed under load, with "BUG: spinlock bad magic" and GPF on 6b6b6b6b6b6b6b6b. Examining the second crash, page_vma_mapped_walk_done()'s spin_unlock of pvmw->ptl (serving page_referenced() on a file THP, that had found a page table at *pmd) discovers that the page table page and its lock have already been freed by the time it comes to unlock. Follow the example of retract_page_tables(), but we only need one of huge page lock or i_mmap_lock_write to secure against this: because it's the narrower lock, and because it simplifies collapse_pte_mapped_thp() to know the hpage earlier, choose to rely on huge page lock here. Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP") Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: Song Liu <songliubraving@xxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> [5.4+] Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008021213070.27773@eggly.anvils Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/khugepaged.c | 44 +++++++++++++++++++------------------------- 1 file changed, 19 insertions(+), 25 deletions(-) --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1294,7 +1294,7 @@ void collapse_pte_mapped_thp(struct mm_s { unsigned long haddr = addr & HPAGE_PMD_MASK; struct vm_area_struct *vma = find_vma(mm, haddr); - struct page *hpage = NULL; + struct page *hpage; pte_t *start_pte, *pte; pmd_t *pmd, _pmd; spinlock_t *ptl; @@ -1314,9 +1314,17 @@ void collapse_pte_mapped_thp(struct mm_s if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE)) return; + hpage = find_lock_page(vma->vm_file->f_mapping, + linear_page_index(vma, haddr)); + if (!hpage) + return; + + if (!PageHead(hpage)) + goto drop_hpage; + pmd = mm_find_pmd(mm, haddr); if (!pmd) - return; + goto drop_hpage; start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl); @@ -1335,30 +1343,11 @@ void collapse_pte_mapped_thp(struct mm_s page = vm_normal_page(vma, addr, *pte); - if (!page || !PageCompound(page)) - goto abort; - - if (!hpage) { - hpage = compound_head(page); - /* - * The mapping of the THP should not change. - * - * Note that uprobe, debugger, or MAP_PRIVATE may - * change the page table, but the new page will - * not pass PageCompound() check. - */ - if (WARN_ON(hpage->mapping != vma->vm_file->f_mapping)) - goto abort; - } - /* - * Confirm the page maps to the correct subpage. - * - * Note that uprobe, debugger, or MAP_PRIVATE may change - * the page table, but the new page will not pass - * PageCompound() check. + * Note that uprobe, debugger, or MAP_PRIVATE may change the + * page table, but the new page will not be a subpage of hpage. */ - if (WARN_ON(hpage + i != page)) + if (hpage + i != page) goto abort; count++; } @@ -1377,7 +1366,7 @@ void collapse_pte_mapped_thp(struct mm_s pte_unmap_unlock(start_pte, ptl); /* step 3: set proper refcount and mm_counters. */ - if (hpage) { + if (count) { page_ref_sub(hpage, count); add_mm_counter(vma->vm_mm, mm_counter_file(hpage), -count); } @@ -1388,10 +1377,15 @@ void collapse_pte_mapped_thp(struct mm_s spin_unlock(ptl); mm_dec_nr_ptes(mm); pte_free(mm, pmd_pgtable(_pmd)); + +drop_hpage: + unlock_page(hpage); + put_page(hpage); return; abort: pte_unmap_unlock(start_pte, ptl); + goto drop_hpage; } static int khugepaged_collapse_pte_mapped_thps(struct mm_slot *mm_slot) Patches currently in stable-queue which might be from hughd@xxxxxxxxxx are queue-5.4/khugepaged-collapse_pte_mapped_thp-flush-the-right-range.patch queue-5.4/khugepaged-collapse_pte_mapped_thp-protect-the-pmd-lock.patch