Messages in this thread Patch in this message | | | Date | Sun, 29 Nov 2009 15:50:32 +0000 (GMT) | From | Hugh Dickins <> | Subject | [PATCH] ksm: hold anon_vma in rmap_item fix |
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KSM mem_cgroup testing oopsed on NULL pointer in mem_cgroup_from_task(), called from the mm_match_cgroup() in page_referenced_ksm().
Right, it is inappropriate to use mm_match_cgroup() on rmap_item->mm there: that mm could be waiting for ksmd's final mmdrop(), with its mm->owner task long gone.
Move the mm_match_cgroup() test down into the anon_vma loop, which is where it now should be to match page_referenced_anon(). The anon_vma guarantees its vmas are valid, which guarantee their mms are valid.
However... although this moves the oops from easy-to-reproduce to never-seen, I think we shall want to do more later: so far as I can see, with or without KSM, the use of mm->owner from page_referenced() is unsafe. No problem when NULL, but it may have been left pointing to a task_struct freed by now, with nonsense in mm->owner->cgroups.
But let's put this patch in while we discuss that separately: perhaps mm_need_new_owner() should not short-circuit when mm_users <= 1, or perhaps it should then set mm->owner to NULL, or perhaps we abandon mm->owner as more trouble than it's worth, or... perhaps I'm wrong.
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> ---
mm/ksm.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
--- mmotm/mm/ksm.c 2009-11-25 09:28:50.000000000 +0000 +++ linux/mm/ksm.c 2009-11-27 13:49:50.000000000 +0000 @@ -1565,9 +1565,6 @@ again: struct anon_vma *anon_vma = rmap_item->anon_vma; struct vm_area_struct *vma; - if (memcg && !mm_match_cgroup(rmap_item->mm, memcg)) - continue; - spin_lock(&anon_vma->lock); list_for_each_entry(vma, &anon_vma->head, anon_vma_node) { if (rmap_item->address < vma->vm_start || @@ -1582,6 +1579,9 @@ again: if ((rmap_item->mm == vma->vm_mm) == search_new_forks) continue; + if (memcg && !mm_match_cgroup(vma->vm_mm, memcg)) + continue; + referenced += page_referenced_one(page, vma, rmap_item->address, &mapcount, vm_flags); if (!search_new_forks || !mapcount)
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