From: Johannes Weiner <hannes@xxxxxxxxxxx>
Subject: mm: memcontrol: fix NULL pointer crash in test_clear_page_writeback()
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
[] RSP: 0018:ffff8e3abfd03d78 EFLAGS: 00010046
[] RAX: 0000000000000000 RBX: ffffdb59c03f8900 RCX: ffffffffffffffe8
[] RDX: 0000000000000000 RSI: 0000000000000010 RDI: ffff8e3abffeb000
[] RBP: ffff8e3abfd03da8 R08: 0000000000020059 R09: 00000000fffffffc
[] R10: 0000000000000000 R11: 0000000000020048 R12: ffff8e3a8c39f668
[] R13: 0000000000000001 R14: ffff8e3a8c39f680 R15: 0000000000000000
[] FS: 0000000000000000(0000) GS:ffff8e3abfd00000(0000) knlGS:0000000000000000
[] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[] CR2: 00000000000003b0 CR3: 000000002c5e1000 CR4: 00000000000406e0
[] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[] 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@xxxxxxxxxxx
Fixes: 62cccb8c8e7a ("mm: simplify lock_page_memcg()")
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Reported-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
Tested-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
Reported-by: Bradley Bolen <bradleybolen@xxxxxxxxx>
Tested-by: Brad Bolen <bradleybolen@xxxxxxxxx>
Cc: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [4.6+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/memcontrol.h | 10 ++++++--
mm/memcontrol.c | 43 +++++++++++++++++++++++++----------
mm/page-writeback.c | 15 +++++++++---
3 files changed, 51 insertions(+), 17 deletions(-)
diff -puN include/linux/memcontrol.h~mm-memcontrol-fix-null-pointer-crash-in-test_clear_page_writeback include/linux/memcontrol.h
--- a/include/linux/memcontrol.h~mm-memcontrol-fix-null-pointer-crash-in-test_clear_page_writeback
+++ a/include/linux/memcontrol.h
@@ -484,7 +484,8 @@ bool mem_cgroup_oom_synchronize(bool wai
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 unsigned long memcg_page_state(struct mem_cgroup *memcg,
@@ -809,7 +810,12 @@ mem_cgroup_print_oom_info(struct mem_cgr
{
}
-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)
{
}
diff -puN mm/memcontrol.c~mm-memcontrol-fix-null-pointer-crash-in-test_clear_page_writeback mm/memcontrol.c
--- a/mm/memcontrol.c~mm-memcontrol-fix-null-pointer-crash-in-test_clear_page_writeback
+++ a/mm/memcontrol.c
@@ -1611,9 +1611,13 @@ cleanup:
* @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;
@@ -1622,18 +1626,24 @@ 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) {
@@ -1649,18 +1659,18 @@ again:
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;
@@ -1672,6 +1682,15 @@ 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 -puN mm/page-writeback.c~mm-memcontrol-fix-null-pointer-crash-in-test_clear_page_writeback mm/page-writeback.c
--- a/mm/page-writeback.c~mm-memcontrol-fix-null-pointer-crash-in-test_clear_page_writeback
+++ a/mm/page-writeback.c
@@ -2724,9 +2724,12 @@ 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;
+ struct lruvec *lruvec;
int ret;
- lock_page_memcg(page);
+ memcg = lock_page_memcg(page);
+ lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
if (mapping && mapping_use_writeback_tags(mapping)) {
struct inode *inode = mapping->host;
struct backing_dev_info *bdi = inode_to_bdi(inode);
@@ -2754,12 +2757,18 @@ int test_clear_page_writeback(struct pag
} 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) {
- dec_lruvec_page_state(page, NR_WRITEBACK);
+ dec_lruvec_state(lruvec, 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;
}
_