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Subject[PATCH 4/3] memcg: call uncharge_swapcache outside of tree_lock (Re: [PATCH 0/3] fix stale swap cache account leak in memcg v7)
On Tue, 12 May 2009 10:44:01 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> I hope this version gets acks..
> ==
> As Nishimura reported, there is a race at handling swap cache.
>
> Typical cases are following (from Nishimura's mail)
>
>
> == Type-1 ==
> If some pages of processA has been swapped out, it calls free_swap_and_cache().
> And if at the same time, processB is calling read_swap_cache_async() about
> a swap entry *that is used by processA*, a race like below can happen.
>
> processA | processB
> -------------------------------------+-------------------------------------
> (free_swap_and_cache()) | (read_swap_cache_async())
> | swap_duplicate()
> | __set_page_locked()
> | add_to_swap_cache()
> swap_entry_free() == 0 |
> find_get_page() -> found |
> try_lock_page() -> fail & return |
> | lru_cache_add_anon()
> | doesn't link this page to memcg's
> | LRU, because of !PageCgroupUsed.
>
> This type of leak can be avoided by setting /proc/sys/vm/page-cluster to 0.
>
>
> == Type-2 ==
> Assume processA is exiting and pte points to a page(!PageSwapCache).
> And processB is trying reclaim the page.
>
> processA | processB
> -------------------------------------+-------------------------------------
> (page_remove_rmap()) | (shrink_page_list())
> mem_cgroup_uncharge_page() |
> ->uncharged because it's not |
> PageSwapCache yet. |
> So, both mem/memsw.usage |
> are decremented. |
> | add_to_swap() -> added to swap cache.
>
> If this page goes thorough without being freed for some reason, this page
> doesn't goes back to memcg's LRU because of !PageCgroupUsed.
>
>
> Considering Type-1, it's better to avoid swapin-readahead when memcg is used.
> swapin-readahead just read swp_entries which are near to requested entry. So,
> pages not to be used can be on memory (on global LRU). When memcg is used,
> this is not good behavior anyway.
>
> Considering Type-2, the page should be freed from SwapCache right after WriteBack.
> Free swapped out pages as soon as possible is a good nature to memcg, anyway.
>
> The patch set includes followng
> [1/3] add mem_cgroup_is_activated() function. which tell us memcg is _really_ used.
> [2/3] fix swap cache handling race by avoidng readahead.
> [3/3] fix swap cache handling race by check swapcount again.
>
> Result is good under my test.
>
These patches seem to work well on my side too.


BTW, we need one more fix which I found in a long term test last week.
After this patch, it survived all through the weekend in my test.

I don't know why we've never hit this bug so far.
I think I hit it because my memcg_free_unused_swapcache() patch increases
the possibility of calling mem_cgroup_uncharge_swapcache().

Thanks,
Daisuke Nishimura.
===
From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

memcg: call mem_cgroup_uncharge_swapcache() outside of tree_lock

It's rare, but I hit a spinlock lockup.

BUG: spinlock lockup on CPU#2, page01/24205, ffffffff806faf18
Pid: 24205, comm: page01 Not tainted 2.6.30-rc4-5845621d #1
Call Trace:
<IRQ> [<ffffffff80294f93>] ? test_clear_page_writeback+0x4d/0xff
[<ffffffff803752bc>] ? _raw_spin_lock+0xfb/0x122
[<ffffffff804ee9ba>] ? _spin_lock_irqsave+0x59/0x70
[<ffffffff80294f93>] ? test_clear_page_writeback+0x4d/0xff
[<ffffffff8029649c>] ? rotate_reclaimable_page+0x87/0x8e
[<ffffffff80294f93>] ? test_clear_page_writeback+0x4d/0xff
[<ffffffff8028d4ed>] ? end_page_writeback+0x1c/0x3d
[<ffffffff802ac4c6>] ? end_swap_bio_write+0x57/0x65
[<ffffffff803516d9>] ? __end_that_request_first+0x1f3/0x2e4
[<ffffffff803515f2>] ? __end_that_request_first+0x10c/0x2e4
[<ffffffff803517e5>] ? end_that_request_data+0x1b/0x4c
[<ffffffff8035225a>] ? blk_end_io+0x1c/0x76
[<ffffffffa0060702>] ? scsi_io_completion+0x1dc/0x467 [scsi_mod]
[<ffffffff80356394>] ? blk_done_softirq+0x66/0x76
[<ffffffff8024002e>] ? __do_softirq+0xae/0x182
[<ffffffff8020cb3c>] ? call_softirq+0x1c/0x2a
[<ffffffff8020de9a>] ? do_softirq+0x31/0x83
[<ffffffff8020d57b>] ? do_IRQ+0xa9/0xbf
[<ffffffff8020c393>] ? ret_from_intr+0x0/0xf
<EOI> [<ffffffff8026fd56>] ? res_counter_uncharge+0x67/0x70
[<ffffffff802bf04a>] ? __mem_cgroup_uncharge_common+0xbd/0x158
[<ffffffff802a0f55>] ? unmap_vmas+0x7ef/0x829
[<ffffffff802a8a3a>] ? page_remove_rmap+0x1b/0x36
[<ffffffff802a0c11>] ? unmap_vmas+0x4ab/0x829
[<ffffffff802a5243>] ? exit_mmap+0xa7/0x11c
[<ffffffff80239009>] ? mmput+0x41/0x9f
[<ffffffff8023cf7b>] ? exit_mm+0x101/0x10c
[<ffffffff8023e481>] ? do_exit+0x1a4/0x61e
[<ffffffff80259391>] ? trace_hardirqs_on_caller+0x11d/0x148
[<ffffffff8023e96e>] ? do_group_exit+0x73/0xa5
[<ffffffff8023e9b2>] ? sys_exit_group+0x12/0x16
[<ffffffff8020b96b>] ? system_call_fastpath+0x16/0x1b
This is caused when:

CPU1: __mem_cgroup_uncharge_common(), which is holding page_cgroup lock,
is interuppted and end_swap_bio_write(), which tries to hold
swapper_space.tree_lock, is called in the interrupt context.
CPU2: mem_cgroup_uncharge_swapcache(), which is called under swapper_space.tree_lock,
is spinning at lock_page_cgroup() in __mem_cgroup_uncharge_common().

IIUC, there is no need that mem_cgroup_uncharge_swapcache() should be called under
swapper_space.tree.lock, so move it outside the tree_lock.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
include/linux/swap.h | 5 +++++
mm/memcontrol.c | 2 +-
mm/swap_state.c | 4 +---
mm/vmscan.c | 1 +
4 files changed, 8 insertions(+), 4 deletions(-)
diff --git a/include/linux/swap.h b/include/linux/swap.h
index caf0767..6ea541d 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -431,6 +431,11 @@ static inline swp_entry_t get_swap_page(void)
#define has_swap_token(x) 0
#define disable_swap_token() do { } while(0)

+static inline void
+mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
+{
+}
+
#endif /* CONFIG_SWAP */
#endif /* __KERNEL__*/
#endif /* _LINUX_SWAP_H */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0c9c1ad..379f200 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1489,7 +1489,7 @@ void mem_cgroup_uncharge_cache_page(struct page *page)
}

/*
- * called from __delete_from_swap_cache() and drop "page" account.
+ * called after __delete_from_swap_cache() and drop "page" account.
* memcg information is recorded to swap_cgroup of "ent"
*/
void mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
diff --git a/mm/swap_state.c b/mm/swap_state.c
index e389ef2..7624c89 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -109,8 +109,6 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
*/
void __delete_from_swap_cache(struct page *page)
{
- swp_entry_t ent = {.val = page_private(page)};
-
VM_BUG_ON(!PageLocked(page));
VM_BUG_ON(!PageSwapCache(page));
VM_BUG_ON(PageWriteback(page));
@@ -121,7 +119,6 @@ void __delete_from_swap_cache(struct page *page)
total_swapcache_pages--;
__dec_zone_page_state(page, NR_FILE_PAGES);
INC_CACHE_INFO(del_total);
- mem_cgroup_uncharge_swapcache(page, ent);
}

/**
@@ -191,6 +188,7 @@ void delete_from_swap_cache(struct page *page)
__delete_from_swap_cache(page);
spin_unlock_irq(&swapper_space.tree_lock);

+ mem_cgroup_uncharge_swapcache(page, entry);
swap_free(entry);
page_cache_release(page);
}
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 337be66..e674cd1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -470,6 +470,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page)
swp_entry_t swap = { .val = page_private(page) };
__delete_from_swap_cache(page);
spin_unlock_irq(&mapping->tree_lock);
+ mem_cgroup_uncharge_swapcache(page, swap);
swap_free(swap);
} else {
__remove_from_page_cache(page);

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