lkml.org 
[lkml]   [2009]   [Nov]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH] ksm: hold anon_vma in rmap_item fix
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)

\
 
 \ /
  Last update: 2009-11-29 16:53    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site