From: Greg Thelen gthelen@google.com Subject: writeback: safer lock nesting
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. Stable 4.4 has minor conflicts applying this patch. For a clean 4.4 patch see "[PATCH for-4.4] writeback: safer lock nesting" https://lkml.org/lkml/2018/4/11/146
Wang Long said "this deadlock occurs three times in our environment"
[gthelen@google.com: v4] Link: http://lkml.kernel.org/r/20180411084653.254724-1-gthelen@google.com [akpm@linux-foundation.org: comment tweaks, struct initialization simplification] Change-Id: Ibb773e8045852978f6207074491d262f1b3fb613 Link: http://lkml.kernel.org/r/20180410005908.167976-1-gthelen@google.com Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates") Signed-off-by: Greg Thelen gthelen@google.com Reported-by: Wang Long wanglong19@meituan.com Acked-by: Wang Long wanglong19@meituan.com Acked-by: Michal Hocko mhocko@suse.com Reviewed-by: Andrew Morton akpm@linux-foundation.org Cc: Johannes Weiner hannes@cmpxchg.org Cc: Tejun Heo tj@kernel.org Cc: Nicholas Piggin npiggin@gmail.com Cc: stable@vger.kernel.org [v4.2+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
fs/fs-writeback.c | 7 +++--- include/linux/backing-dev-defs.h | 5 ++++ include/linux/backing-dev.h | 30 +++++++++++++++-------------- mm/page-writeback.c | 18 ++++++++--------- 4 files changed, 34 insertions(+), 26 deletions(-)
diff -puN fs/fs-writeback.c~writeback-safer-lock-nesting fs/fs-writeback.c --- a/fs/fs-writeback.c~writeback-safer-lock-nesting +++ a/fs/fs-writeback.c @@ -745,11 +745,12 @@ int inode_congested(struct inode *inode, */ 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 -puN include/linux/backing-dev-defs.h~writeback-safer-lock-nesting include/linux/backing-dev-defs.h --- a/include/linux/backing-dev-defs.h~writeback-safer-lock-nesting +++ a/include/linux/backing-dev-defs.h @@ -223,6 +223,11 @@ static inline void set_bdi_congested(str set_wb_congested(bdi->wb.congested, sync); }
+struct wb_lock_cookie { + bool locked; + unsigned long flags; +}; + #ifdef CONFIG_CGROUP_WRITEBACK
/** diff -puN include/linux/backing-dev.h~writeback-safer-lock-nesting include/linux/backing-dev.h --- a/include/linux/backing-dev.h~writeback-safer-lock-nesting +++ a/include/linux/backing-dev.h @@ -347,7 +347,7 @@ static inline struct bdi_writeback *inod /** * 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, the i_pages lock or wb->list_lock. This @@ -355,12 +355,12 @@ static inline struct bdi_writeback *inod * 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();
@@ -368,10 +368,10 @@ unlocked_inode_to_wb_begin(struct inode * 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)) - xa_lock_irq(&inode->i_mapping->i_pages); + if (unlikely(cookie->locked)) + xa_lock_irqsave(&inode->i_mapping->i_pages, cookie->flags);
/* * Protected by either !I_WB_SWITCH + rcu_read_lock() or the i_pages @@ -383,12 +383,13 @@ unlocked_inode_to_wb_begin(struct inode /** * 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)) - xa_unlock_irq(&inode->i_mapping->i_pages); + if (unlikely(cookie->locked)) + xa_unlock_irqrestore(&inode->i_mapping->i_pages, cookie->flags);
rcu_read_unlock(); } @@ -435,12 +436,13 @@ static inline struct bdi_writeback *inod }
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 -puN mm/page-writeback.c~writeback-safer-lock-nesting mm/page-writeback.c --- a/mm/page-writeback.c~writeback-safer-lock-nesting +++ a/mm/page-writeback.c @@ -2502,13 +2502,13 @@ void account_page_redirty(struct page *p 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_node_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); @@ -2614,15 +2614,15 @@ void __cancel_dirty_page(struct page *pa if (mapping_cap_account_dirty(mapping)) { struct inode *inode = mapping->host; struct bdi_writeback *wb; - bool locked; + struct wb_lock_cookie cookie = {};
lock_page_memcg(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, wb);
- unlocked_inode_to_wb_end(inode, locked); + unlocked_inode_to_wb_end(inode, &cookie); unlock_page_memcg(page); } else { ClearPageDirty(page); @@ -2654,7 +2654,7 @@ int clear_page_dirty_for_io(struct page if (mapping && mapping_cap_account_dirty(mapping)) { struct inode *inode = mapping->host; struct bdi_writeback *wb; - bool locked; + struct wb_lock_cookie cookie = {};
/* * Yes, Virginia, this is indeed insane. @@ -2691,14 +2691,14 @@ int clear_page_dirty_for_io(struct page * always locked coming in here, so we get the desired * exclusion. */ - wb = unlocked_inode_to_wb_begin(inode, &locked); + wb = unlocked_inode_to_wb_begin(inode, &cookie); if (TestClearPageDirty(page)) { dec_lruvec_page_state(page, NR_FILE_DIRTY); dec_zone_page_state(page, NR_ZONE_WRITE_PENDING); dec_wb_stat(wb, WB_RECLAIMABLE); ret = 1; } - unlocked_inode_to_wb_end(inode, locked); + unlocked_inode_to_wb_end(inode, &cookie); return ret; } return TestClearPageDirty(page); _