From 4829b906cc063cb7cd1b7f34fa05de6db75ec8bb Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Sat, 2 Oct 2010 17:46:06 -0700 Subject: [PATCH] ksm: fix page_address_in_vma anon_vma oops 2.6.36-rc1 commit 21d0d443cdc1658a8c1484fdcece4803f0f96d0e "rmap: resurrect page_address_in_vma anon_vma check" was right to resurrect that check; but now that it's comparing anon_vma->roots instead of just anon_vmas, there's a danger of oopsing on a NULL anon_vma. In most cases no NULL anon_vma ever gets here; but it turns out that occasionally KSM, when enabled on a forked or forking process, will itself call page_address_in_vma() on a "half-KSM" page left over from an earlier failed attempt to merge - whose page_anon_vma() is NULL. It's my bug that those should be getting here at all: I thought they were already dealt with, this oops proves me wrong, I'll fix it in the next release - such pages are effectively pinned until their process exits, since rmap cannot find their ptes (though swapoff can). For now just work around it by making page_address_in_vma() safe (and add a comment on why that check is wanted anyway). A similar check in __page_check_anon_rmap() is safe because do_page_add_anon_rmap() already excluded KSM pages. Signed-off-by: Hugh Dickins Cc: Andrew Morton Cc: Andrea Arcangeli Cc: Rik van Riel Signed-off-by: Linus Torvalds --- mm/rmap.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/mm/rmap.c b/mm/rmap.c index 9d2ba01bd4f..92e6757f196 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -381,7 +381,13 @@ vma_address(struct page *page, struct vm_area_struct *vma) unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma) { if (PageAnon(page)) { - if (vma->anon_vma->root != page_anon_vma(page)->root) + struct anon_vma *page__anon_vma = page_anon_vma(page); + /* + * Note: swapoff's unuse_vma() is more efficient with this + * check, and needs it to match anon_vma when KSM is active. + */ + if (!vma->anon_vma || !page__anon_vma || + vma->anon_vma->root != page__anon_vma->root) return -EFAULT; } else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) { if (!vma->vm_file || -- 2.39.3