From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> Subject: thp: fix corner case of munlock() of PTE-mapped THPs The following program triggers BUG() in munlock_vma_pages_range(): // autogenerated by syzkaller (http://github.com/google/syzkaller) #include <sys/mman.h> int main() { mmap((void*)0x20105000ul, 0xc00000ul, 0x2ul, 0x2172ul, -1, 0); mremap((void*)0x201fd000ul, 0x4000ul, 0xc00000ul, 0x3ul, 0x203f0000ul); return 0; } The test-case constructs the situation when munlock_vma_pages_range() finds PTE-mapped THP-head in the middle of page table and, by mistake, skips HPAGE_PMD_NR pages after that. As result, on the next iteration it hits the middle of PMD-mapped THP and gets upset seeing mlocked tail page. The solution is only skip HPAGE_PMD_NR pages if the THP was mlocked during munlock_vma_page(). It would guarantee that the page is PMD-mapped as we never mlock PTE-mapeed THPs. Fixes: e90309c9f772 ("thp: allow mlocked THP again") Link: http://lkml.kernel.org/r/20161115132703.7s7rrgmwttegcdh4@xxxxxxxxxxxxxxxxxx Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> Cc: syzkaller <syzkaller@xxxxxxxxxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> [4.5+] Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/mlock.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff -puN mm/mlock.c~mm-bug-in-munlock_vma_pages_range mm/mlock.c --- a/mm/mlock.c~mm-bug-in-munlock_vma_pages_range +++ a/mm/mlock.c @@ -190,10 +190,13 @@ unsigned int munlock_vma_page(struct pag */ spin_lock_irq(zone_lru_lock(zone)); - nr_pages = hpage_nr_pages(page); - if (!TestClearPageMlocked(page)) + if (!TestClearPageMlocked(page)) { + /* Potentially, PTE-mapped THP: do not skip the rest PTEs */ + nr_pages = 1; goto unlock_out; + } + nr_pages = hpage_nr_pages(page); __mod_zone_page_state(zone, NR_MLOCK, -nr_pages); if (__munlock_isolate_lru_page(page, true)) { _ -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html