From: Johannes Weiner hannes@cmpxchg.org

commit 739f79fc9db1b38f96b5a5109b247a650fbebf6d upstream

Jaegeuk and Brad report a NULL pointer crash when writeback ending tries to update the memcg stats:

BUG: unable to handle kernel NULL pointer dereference at 00000000000003b0 IP: test_clear_page_writeback+0x12e/0x2c0 [...] RIP: 0010:test_clear_page_writeback+0x12e/0x2c0 Call Trace: <IRQ> end_page_writeback+0x47/0x70 f2fs_write_end_io+0x76/0x180 [f2fs] bio_endio+0x9f/0x120 blk_update_request+0xa8/0x2f0 scsi_end_request+0x39/0x1d0 scsi_io_completion+0x211/0x690 scsi_finish_command+0xd9/0x120 scsi_softirq_done+0x127/0x150 __blk_mq_complete_request_remote+0x13/0x20 flush_smp_call_function_queue+0x56/0x110 generic_smp_call_function_single_interrupt+0x13/0x30 smp_call_function_single_interrupt+0x27/0x40 call_function_single_interrupt+0x89/0x90 RIP: 0010:native_safe_halt+0x6/0x10

(gdb) l *(test_clear_page_writeback+0x12e) 0xffffffff811bae3e is in test_clear_page_writeback (./include/linux/memcontrol.h:619). 614 mod_node_page_state(page_pgdat(page), idx, val); 615 if (mem_cgroup_disabled() || !page->mem_cgroup) 616 return; 617 mod_memcg_state(page->mem_cgroup, idx, val); 618 pn = page->mem_cgroup->nodeinfo[page_to_nid(page)]; 619 this_cpu_add(pn->lruvec_stat->count[idx], val); 620 } 621 622 unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order, 623 gfp_t gfp_mask,

The issue is that writeback doesn't hold a page reference and the page might get freed after PG_writeback is cleared (and the mapping is unlocked) in test_clear_page_writeback(). The stat functions looking up the page's node or zone are safe, as those attributes are static across allocation and free cycles. But page->mem_cgroup is not, and it will get cleared if we race with truncation or migration.

It appears this race window has been around for a while, but less likely to trigger when the memcg stats were updated first thing after PG_writeback is cleared. Recent changes reshuffled this code to update the global node stats before the memcg ones, though, stretching the race window out to an extent where people can reproduce the problem.

Update test_clear_page_writeback() to look up and pin page->mem_cgroup before clearing PG_writeback, then not use that pointer afterward. It is a partial revert of 62cccb8c8e7a ("mm: simplify lock_page_memcg()") but leaves the pageref-holding callsites that aren't affected alone.

Link: http://lkml.kernel.org/r/20170809183825.GA26387@cmpxchg.org Fixes: 62cccb8c8e7a ("mm: simplify lock_page_memcg()") Signed-off-by: Johannes Weiner hannes@cmpxchg.org Reported-by: Jaegeuk Kim jaegeuk@kernel.org Tested-by: Jaegeuk Kim jaegeuk@kernel.org Reported-by: Bradley Bolen bradleybolen@gmail.com Tested-by: Brad Bolen bradleybolen@gmail.com Cc: Vladimir Davydov vdavydov@virtuozzo.com Cc: Michal Hocko mhocko@suse.cz Cc: stable@vger.kernel.org [4.6+] Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org [guptap@codeaurora.org: Resolved merge conflicts] Signed-off-by: Prakash Gupta guptap@codeaurora.org Signed-off-by: Florian Fainelli f.fainelli@gmail.com --- This patch is present in a downstream Android tree:

https://source.mcwhirter.io/craige/bluecross/commit/d4a742865c6b69ef93169474...

and I happened to have stumbled across the same problem too.

Johannes can you review it for correctness with respect to the 4.9 kernel? Thanks!

include/linux/memcontrol.h | 33 ++++++++++++++++++++++++----- mm/memcontrol.c | 43 +++++++++++++++++++++++++++----------- mm/page-writeback.c | 14 ++++++++++--- 3 files changed, 70 insertions(+), 20 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index 8b35bdbdc214..fd77f8303ab9 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -490,9 +490,21 @@ bool mem_cgroup_oom_synchronize(bool wait); extern int do_swap_account; #endif

-void lock_page_memcg(struct page *page); +struct mem_cgroup *lock_page_memcg(struct page *page); +void __unlock_page_memcg(struct mem_cgroup *memcg); void unlock_page_memcg(struct page *page);

+static inline void __mem_cgroup_update_page_stat(struct page *page, + struct mem_cgroup *memcg, + enum mem_cgroup_stat_index idx, + int val) +{ + VM_BUG_ON(!(rcu_read_lock_held() || PageLocked(page))); + + if (memcg && memcg->stat) + this_cpu_add(memcg->stat->count[idx], val); +} + /** * mem_cgroup_update_page_stat - update page state statistics * @page: the page @@ -508,13 +520,12 @@ void unlock_page_memcg(struct page *page); * mem_cgroup_update_page_stat(page, state, -1); * unlock_page(page) or unlock_page_memcg(page) */ + static inline void mem_cgroup_update_page_stat(struct page *page, enum mem_cgroup_stat_index idx, int val) { - VM_BUG_ON(!(rcu_read_lock_held() || PageLocked(page)));

- if (page->mem_cgroup) - this_cpu_add(page->mem_cgroup->stat->count[idx], val); + __mem_cgroup_update_page_stat(page, page->mem_cgroup, idx, val); }

static inline void mem_cgroup_inc_page_stat(struct page *page, @@ -709,7 +720,12 @@ mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p) { }

-static inline void lock_page_memcg(struct page *page) +static inline struct mem_cgroup *lock_page_memcg(struct page *page) +{ + return NULL; +} + +static inline void __unlock_page_memcg(struct mem_cgroup *memcg) { }

@@ -745,6 +761,13 @@ static inline void mem_cgroup_update_page_stat(struct page *page, { }

+static inline void __mem_cgroup_update_page_stat(struct page *page, + struct mem_cgroup *memcg, + enum mem_cgroup_stat_index idx, + int nr) +{ +} + static inline void mem_cgroup_inc_page_stat(struct page *page, enum mem_cgroup_stat_index idx) { diff --git a/mm/memcontrol.c b/mm/memcontrol.c index d4232744c59f..27b0b4f03fcd 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1638,9 +1638,13 @@ bool mem_cgroup_oom_synchronize(bool handle) * @page: the page * * This function protects unlocked LRU pages from being moved to - * another cgroup and stabilizes their page->mem_cgroup binding. + * another cgroup. + * + * It ensures lifetime of the returned memcg. Caller is responsible + * for the lifetime of the page; __unlock_page_memcg() is available + * when @page might get freed inside the locked section. */ -void lock_page_memcg(struct page *page) +struct mem_cgroup *lock_page_memcg(struct page *page) { struct mem_cgroup *memcg; unsigned long flags; @@ -1649,18 +1653,24 @@ void lock_page_memcg(struct page *page) * The RCU lock is held throughout the transaction. The fast * path can get away without acquiring the memcg->move_lock * because page moving starts with an RCU grace period. - */ + * + * The RCU lock also protects the memcg from being freed when + * the page state that is going to change is the only thing + * preventing the page itself from being freed. E.g. writeback + * doesn't hold a page reference and relies on PG_writeback to + * keep off truncation, migration and so forth. + */ rcu_read_lock();

if (mem_cgroup_disabled()) - return; + return NULL; again: memcg = page->mem_cgroup; if (unlikely(!memcg)) - return; + return NULL;

if (atomic_read(&memcg->moving_account) <= 0) - return; + return memcg;

spin_lock_irqsave(&memcg->move_lock, flags); if (memcg != page->mem_cgroup) { @@ -1676,18 +1686,18 @@ void lock_page_memcg(struct page *page) memcg->move_lock_task = current; memcg->move_lock_flags = flags;

- return; + return memcg; } EXPORT_SYMBOL(lock_page_memcg);

/** - * unlock_page_memcg - unlock a page->mem_cgroup binding - * @page: the page + * __unlock_page_memcg - unlock and unpin a memcg + * @memcg: the memcg + * + * Unlock and unpin a memcg returned by lock_page_memcg(). */ -void unlock_page_memcg(struct page *page) +void __unlock_page_memcg(struct mem_cgroup *memcg) { - struct mem_cgroup *memcg = page->mem_cgroup; - if (memcg && memcg->move_lock_task == current) { unsigned long flags = memcg->move_lock_flags;

@@ -1699,6 +1709,15 @@ void unlock_page_memcg(struct page *page)

rcu_read_unlock(); } + +/** + * unlock_page_memcg - unlock a page->mem_cgroup binding + * @page: the page + */ +void unlock_page_memcg(struct page *page) +{ + __unlock_page_memcg(page->mem_cgroup); +} EXPORT_SYMBOL(unlock_page_memcg);

/* diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 462c778b9fb5..498c924f2fcd 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -2717,9 +2717,10 @@ EXPORT_SYMBOL(clear_page_dirty_for_io); int test_clear_page_writeback(struct page *page) { struct address_space *mapping = page_mapping(page); + struct mem_cgroup *memcg; int ret;

- lock_page_memcg(page); + memcg = lock_page_memcg(page); if (mapping && mapping_use_writeback_tags(mapping)) { struct inode *inode = mapping->host; struct backing_dev_info *bdi = inode_to_bdi(inode); @@ -2747,13 +2748,20 @@ int test_clear_page_writeback(struct page *page) } else { ret = TestClearPageWriteback(page); } + /* + * NOTE: Page might be free now! Writeback doesn't hold a page + * reference on its own, it relies on truncation to wait for + * the clearing of PG_writeback. The below can only access + * page state that is static across allocation cycles. + */ if (ret) { - mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_WRITEBACK); + __mem_cgroup_update_page_stat(page, memcg, + MEM_CGROUP_STAT_WRITEBACK, -1); dec_node_page_state(page, NR_WRITEBACK); dec_zone_page_state(page, NR_ZONE_WRITE_PENDING); inc_node_page_state(page, NR_WRITTEN); } - unlock_page_memcg(page); + __unlock_page_memcg(memcg); return ret; }