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 <hughd@xxxxxxxxxx> --- mm/rmap.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) --- 2.6.36-rc6/mm/rmap.c 2010-09-28 22:42:43.000000000 -0700 +++ linux/mm/rmap.c 2010-09-28 23:27:05.000000000 -0700 @@ -381,7 +381,13 @@ vma_address(struct page *page, struct vm 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 || -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>