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author | Hugh Dickins <hugh@veritas.com> | 2008-03-04 14:29:16 -0800 |
---|---|---|
committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2008-03-04 16:35:15 -0800 |
commit | fb59e9f1e9786635ea12e12bf6adbb132e10f979 (patch) | |
tree | 9104d67895e0e85e3014c60389146ab2318faa7c | |
parent | 9b3c0a07e0fca35e36751680de3e4c76dbff5df3 (diff) | |
download | kernel_samsung_smdk4412-fb59e9f1e9786635ea12e12bf6adbb132e10f979.zip kernel_samsung_smdk4412-fb59e9f1e9786635ea12e12bf6adbb132e10f979.tar.gz kernel_samsung_smdk4412-fb59e9f1e9786635ea12e12bf6adbb132e10f979.tar.bz2 |
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.c | 17 |
1 files changed, 9 insertions, 8 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index f720670..8b9f6ca 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); } /* |