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authorHugh Dickins <hugh@veritas.com>2008-03-04 14:29:16 -0800
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2008-03-04 16:35:15 -0800
commitfb59e9f1e9786635ea12e12bf6adbb132e10f979 (patch)
tree9104d67895e0e85e3014c60389146ab2318faa7c
parent9b3c0a07e0fca35e36751680de3e4c76dbff5df3 (diff)
downloadkernel-fb59e9f1e9786635ea12e12bf6adbb132e10f979.tar.gz
memcg: fix oops on NULL lru list
While testing force_empty, during an exit_mmap, __mem_cgroup_remove_list called from mem_cgroup_uncharge_page oopsed on a NULL pointer in the lru list. I couldn't see what racing tasks on other cpus were doing, but surmise that another must have been in mem_cgroup_charge_common on the same page, between its unlock_page_cgroup and spin_lock_irqsave near done (thanks to that kzalloc which I'd almost changed to a kmalloc). Normally such a race cannot happen, the ref_cnt prevents it, the final uncharge cannot race with the initial charge. But force_empty buggers the ref_cnt, that's what it's all about; and thereafter forced pages are vulnerable to races such as this (just think of a shared page also mapped into an mm of another mem_cgroup than that just emptied). And remain vulnerable until they're freed indefinitely later. This patch just fixes the oops by moving the unlock_page_cgroups down below adding to and removing from the list (only possible given the previous patch); and while we're at it, we might as well make it an invariant that page->page_cgroup is always set while pc is on lru. But this behaviour of force_empty seems highly unsatisfactory to me: why have a ref_cnt if we always have to cope with it being violated (as in the earlier page migration patch). We may prefer force_empty to move pages to an orphan mem_cgroup (could be the root, but better not), from which other cgroups could recover them; we might need to reverse the locking again; but no time now for such concerns. Signed-off-by: Hugh Dickins <hugh@veritas.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/memcontrol.c17
1 files changed, 9 insertions, 8 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f72067094e05..8b9f6cae938e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -623,13 +623,13 @@ retry:
goto retry;
}
page_assign_page_cgroup(page, pc);
- unlock_page_cgroup(page);
mz = page_cgroup_zoneinfo(pc);
spin_lock_irqsave(&mz->lru_lock, flags);
__mem_cgroup_add_list(pc);
spin_unlock_irqrestore(&mz->lru_lock, flags);
+ unlock_page_cgroup(page);
done:
return 0;
out:
@@ -677,14 +677,14 @@ void mem_cgroup_uncharge_page(struct page *page)
VM_BUG_ON(pc->ref_cnt <= 0);
if (--(pc->ref_cnt) == 0) {
- page_assign_page_cgroup(page, NULL);
- unlock_page_cgroup(page);
-
mz = page_cgroup_zoneinfo(pc);
spin_lock_irqsave(&mz->lru_lock, flags);
__mem_cgroup_remove_list(pc);
spin_unlock_irqrestore(&mz->lru_lock, flags);
+ page_assign_page_cgroup(page, NULL);
+ unlock_page_cgroup(page);
+
mem = pc->mem_cgroup;
res_counter_uncharge(&mem->res, PAGE_SIZE);
css_put(&mem->css);
@@ -736,23 +736,24 @@ void mem_cgroup_page_migration(struct page *page, struct page *newpage)
return;
}
- page_assign_page_cgroup(page, NULL);
- unlock_page_cgroup(page);
-
mz = page_cgroup_zoneinfo(pc);
spin_lock_irqsave(&mz->lru_lock, flags);
__mem_cgroup_remove_list(pc);
spin_unlock_irqrestore(&mz->lru_lock, flags);
+ page_assign_page_cgroup(page, NULL);
+ unlock_page_cgroup(page);
+
pc->page = newpage;
lock_page_cgroup(newpage);
page_assign_page_cgroup(newpage, pc);
- unlock_page_cgroup(newpage);
mz = page_cgroup_zoneinfo(pc);
spin_lock_irqsave(&mz->lru_lock, flags);
__mem_cgroup_add_list(pc);
spin_unlock_irqrestore(&mz->lru_lock, flags);
+
+ unlock_page_cgroup(newpage);
}
/*