The patch titled Subject: writeback: safer lock nesting has been added to the -mm tree. Its filename is writeback-safer-lock-nesting.patch

This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/writeback-safer-lock-nesting.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/writeback-safer-lock-nesting.patch

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------------------------------------------------------ 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

Given the deadlock is not currently possible, it's debatable if there's any reason to modify the kernel. I suggest we should to prevent future surprises.

Wang Long said "this deadlock occurs three times in our environment"

Change-Id: Ibb773e8045852978f6207074491d262f1b3fb613 Link: http://lkml.kernel.org/r/20180410005908.167976-1-gthelen@google.com Signed-off-by: Greg Thelen gthelen@google.com Reported-by: Wang Long wanglong19@meituan.com Acked-by: Wang Long wanglong19@meituan.com Reviewed-by: Andrew Morton akpm@linux-foundation.org Cc: Wang Long wanglong19@meituan.com Cc: Michal Hocko mhocko@kernel.org Cc: Johannes Weiner hannes@cmpxchg.org Cc: Tejun Heo tj@kernel.org Cc: Nicholas Piggin npiggin@gmail.com Cc: stable@vger.kernel.org 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 @@ -346,7 +346,7 @@ 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, mapping->tree_lock or wb->list_lock. This @@ -354,12 +354,11 @@ 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 transaction. IRQ 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();

@@ -367,10 +366,10 @@ 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)) - 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. @@ -382,12 +381,14 @@ 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)) - 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(); } @@ -434,12 +435,13 @@ 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 @@ -2501,13 +2501,13 @@ 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 = {0};

- 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); @@ -2613,15 +2613,15 @@ 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 = {0};

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); @@ -2653,7 +2653,7 @@ 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 = {0};

/* * Yes, Virginia, this is indeed insane. @@ -2690,14 +2690,14 @@ 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); _

Patches currently in -mm which might be from gthelen@google.com are

writeback-safer-lock-nesting.patch