Re: [PATCH for-4.4] writeback: safer lock nesting

On Wed, Apr 11, 2018 at 1:45 AM Greg Thelen gthelen@google.com wrote:

lock_page_memcg()/unlock_page_memcg() use spin_lock_irqsave/restore() if the page's memcg is undergoing move accounting, which occurs when a process leaves its memcg for a new one that has memory.move_charge_at_immigrate set.

unlocked_inode_to_wb_begin,end() use spin_lock_irq/spin_unlock_irq() if

the

given inode is switching writeback domains. Switches occur when enough writes are issued from a new domain.

This existing pattern is thus suspicious: lock_page_memcg(page); unlocked_inode_to_wb_begin(inode, &locked); ... unlocked_inode_to_wb_end(inode, locked); unlock_page_memcg(page);

If both inode switch and process memcg migration are both in-flight then unlocked_inode_to_wb_end() will unconditionally enable interrupts while still holding the lock_page_memcg() irq spinlock. This suggests the possibility of deadlock if an interrupt occurs before unlock_page_memcg().

 truncate
 __cancel_dirty_page
 lock_page_memcg
 unlocked_inode_to_wb_begin
 unlocked_inode_to_wb_end
 <interrupts mistakenly enabled>
                                 <interrupt>
                                 end_page_writeback
                                 test_clear_page_writeback
                                 lock_page_memcg
                                 <deadlock>
 unlock_page_memcg

Due to configuration limitations this deadlock is not currently possible because we don't mix cgroup writeback (a cgroupv2 feature) and memory.move_charge_at_immigrate (a cgroupv1 feature).

If the kernel is hacked to always claim inode switching and memcg moving_account, then this script triggers lockup in less than a minute: cd /mnt/cgroup/memory mkdir a b echo 1 > a/memory.move_charge_at_immigrate echo 1 > b/memory.move_charge_at_immigrate ( echo $BASHPID > a/cgroup.procs while true; do dd if=/dev/zero of=/mnt/big bs=1M count=256 done ) & while true; do sync done & sleep 1h & SLEEP=$! while true; do echo $SLEEP > a/cgroup.procs echo $SLEEP > b/cgroup.procs done

The deadlock does not seem possible, so it's debatable if there's any reason to modify the kernel. I suggest we should to prevent future surprises. And Wang Long said "this deadlock occurs three times in our environment", so there's more reason to apply this, even to stable.

Wang Long: I wasn't able to reproduce the 4.4 problem. But tracing suggests this 4.4 patch is effective. If you can reproduce the problem in your 4.4 environment, then it'd be nice to confirm this fixes it. Thanks!

[ This patch is only for 4.4 stable. Newer stable kernels should use be

able to

cherry pick the upstream "writeback: safer lock nesting" patch. ]

Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb

transaction and use it for stat updates")

Cc: stable@vger.kernel.org # v4.2+ Reported-by: Wang Long wanglong19@meituan.com Signed-off-by: Greg Thelen gthelen@google.com Acked-by: Michal Hocko mhocko@suse.com Acked-by: Wang Long wanglong19@meituan.com

---

fs/fs-writeback.c | 7 ++++--- include/linux/backing-dev-defs.h | 5 +++++ include/linux/backing-dev.h | 31 +++++++++++++++++-------------- mm/page-writeback.c | 18 +++++++++--------- 4 files changed, 35 insertions(+), 26 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 22b30249fbcb..0fe667875852 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -747,11 +747,12 @@ int inode_congested(struct inode *inode, int

cong_bits)

      */
     if (inode && inode_to_wb_is_valid(inode)) {
             struct bdi_writeback *wb;

•           bool locked, congested;
  

•           struct wb_lock_cookie lock_cookie = {};
  

•           bool congested;
  

•           wb = unlocked_inode_to_wb_begin(inode, &locked);
  

•           wb = unlocked_inode_to_wb_begin(inode, &lock_cookie);
             congested = wb_congested(wb, cong_bits);
  

•           unlocked_inode_to_wb_end(inode, locked);
  

•           unlocked_inode_to_wb_end(inode, &lock_cookie);
             return congested;
     }
  

diff --git a/include/linux/backing-dev-defs.h

b/include/linux/backing-dev-defs.h

index 140c29635069..a307c37c2e6c 100644 --- a/include/linux/backing-dev-defs.h +++ b/include/linux/backing-dev-defs.h @@ -191,6 +191,11 @@ static inline void set_bdi_congested(struct

backing_dev_info *bdi, int sync)

     set_wb_congested(bdi->wb.congested, sync);

}

+struct wb_lock_cookie {

•   bool locked;
  

•   unsigned long flags;
  

+};

• #ifdef CONFIG_CGROUP_WRITEBACK

/** diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h index 89d3de3e096b..361274ce5815 100644 --- a/include/linux/backing-dev.h +++ b/include/linux/backing-dev.h @@ -366,7 +366,7 @@ static inline struct bdi_writeback

*inode_to_wb(struct inode *inode)

/**

• unlocked_inode_to_wb_begin - begin unlocked inode wb access

transaction

• @inode: target inode

• • @lockedp: temp bool output param, to be passed to the end function

• • @cookie: output param, to be passed to the end function
  • The caller wants to access the wb associated with @inode but isn't
  • holding inode->i_lock, mapping->tree_lock or wb->list_lock. This

@@ -374,12 +374,12 @@ static inline struct bdi_writeback

*inode_to_wb(struct inode *inode)

• association doesn't change until the transaction is finished with
• unlocked_inode_to_wb_end().

• • The caller must call unlocked_inode_to_wb_end() with *@lockdep
• • afterwards and can't sleep during transaction. IRQ may or may not be
• • disabled on return.

• • The caller must call unlocked_inode_to_wb_end() with *@cookie

afterwards and

• • can't sleep during the transaction. IRQs may or may not be disabled

on

• • return.
  */ static inline struct bdi_writeback *

-unlocked_inode_to_wb_begin(struct inode *inode, bool *lockedp) +unlocked_inode_to_wb_begin(struct inode *inode, struct wb_lock_cookie

*cookie)

{ rcu_read_lock();

@@ -387,10 +387,10 @@ unlocked_inode_to_wb_begin(struct inode *inode,

bool *lockedp)

      * Paired with store_release in inode_switch_wb_work_fn() and
      * ensures that we see the new wb if we see cleared I_WB_SWITCH.
      */

•   *lockedp = smp_load_acquire(&inode->i_state) & I_WB_SWITCH;
  

•   cookie->locked = smp_load_acquire(&inode->i_state) & I_WB_SWITCH;
  

•   if (unlikely(*lockedp))
  

•           spin_lock_irq(&inode->i_mapping->tree_lock);
  

•   if (unlikely(cookie->locked))
  

•           spin_lock_irqsave(&inode->i_mapping->tree_lock,
  

cookie->flags);

     /*
      * Protected by either !I_WB_SWITCH + rcu_read_lock() or

tree_lock.

@@ -402,12 +402,14 @@ unlocked_inode_to_wb_begin(struct inode *inode,

bool *lockedp)

/**

• unlocked_inode_to_wb_end - end inode wb access transaction
• @inode: target inode

• • @locked: *@lockedp from unlocked_inode_to_wb_begin()

• • @cookie: @cookie from unlocked_inode_to_wb_begin()
  */

-static inline void unlocked_inode_to_wb_end(struct inode *inode, bool

locked)

+static inline void unlocked_inode_to_wb_end(struct inode *inode,

•                                       struct wb_lock_cookie *cookie)
  

  {

•   if (unlikely(locked))
  

•           spin_unlock_irq(&inode->i_mapping->tree_lock);
  

•   if (unlikely(cookie->locked))
  

•           spin_unlock_irqrestore(&inode->i_mapping->tree_lock,
  

•                                  cookie->flags);
  

     rcu_read_unlock();

} @@ -454,12 +456,13 @@ static inline struct bdi_writeback

*inode_to_wb(struct inode *inode)

}

static inline struct bdi_writeback * -unlocked_inode_to_wb_begin(struct inode *inode, bool *lockedp) +unlocked_inode_to_wb_begin(struct inode *inode, struct wb_lock_cookie

*cookie)

{ return inode_to_wb(inode); }

-static inline void unlocked_inode_to_wb_end(struct inode *inode, bool

locked)

+static inline void unlocked_inode_to_wb_end(struct inode *inode,

•                                       struct wb_lock_cookie *cookie)
  

  { }

diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 6d0dbde4503b..3309dbda7ffa 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -2510,13 +2510,13 @@ void account_page_redirty(struct page *page) if (mapping && mapping_cap_account_dirty(mapping)) { struct inode *inode = mapping->host; struct bdi_writeback *wb;

•           bool locked;
  

•           struct wb_lock_cookie cookie = {};
  

•           wb = unlocked_inode_to_wb_begin(inode, &locked);
  

•           wb = unlocked_inode_to_wb_begin(inode, &cookie);
             current->nr_dirtied--;
             dec_zone_page_state(page, NR_DIRTIED);
             dec_wb_stat(wb, WB_DIRTIED);
  

•           unlocked_inode_to_wb_end(inode, locked);
  

•           unlocked_inode_to_wb_end(inode, &cookie);
     }
  

  } EXPORT_SYMBOL(account_page_redirty);

@@ -2622,15 +2622,15 @@ void cancel_dirty_page(struct page *page) struct inode *inode = mapping->host; struct bdi_writeback *wb; struct mem_cgroup *memcg;

•           bool locked;
  

•           struct wb_lock_cookie cookie = {};
  

             memcg = mem_cgroup_begin_page_stat(page);

•           wb = unlocked_inode_to_wb_begin(inode, &locked);
  

•           wb = unlocked_inode_to_wb_begin(inode, &cookie);
  

             if (TestClearPageDirty(page))
                     account_page_cleaned(page, mapping, memcg, wb);

•           unlocked_inode_to_wb_end(inode, locked);
  

•           unlocked_inode_to_wb_end(inode, &cookie);
             mem_cgroup_end_page_stat(memcg);
     } else {
             ClearPageDirty(page);
  

@@ -2663,7 +2663,7 @@ int clear_page_dirty_for_io(struct page *page) struct inode *inode = mapping->host; struct bdi_writeback *wb; struct mem_cgroup *memcg;

•           bool locked;
  

•           struct wb_lock_cookie cookie = {};
  

             /*
              * Yes, Virginia, this is indeed insane.

@@ -2701,14 +2701,14 @@ int clear_page_dirty_for_io(struct page *page) * exclusion. */ memcg = mem_cgroup_begin_page_stat(page);

•           wb = unlocked_inode_to_wb_begin(inode, &locked);
  

•           wb = unlocked_inode_to_wb_begin(inode, &cookie);
             if (TestClearPageDirty(page)) {
                     mem_cgroup_dec_page_stat(memcg,
  

MEM_CGROUP_STAT_DIRTY);

                     dec_zone_page_state(page, NR_FILE_DIRTY);
                     dec_wb_stat(wb, WB_RECLAIMABLE);
                     ret = 1;
             }

•           unlocked_inode_to_wb_end(inode, locked);
  

•           unlocked_inode_to_wb_end(inode, &cookie);
             mem_cgroup_end_page_stat(memcg);
             return ret;
     }
  

-- 2.17.0.484.g0c8726318c-goog