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Message-ID: <20180410063357.GS21835@dhcp22.suse.cz>
Date: Tue, 10 Apr 2018 08:33:57 +0200
From: Michal Hocko <mhocko@...nel.org>
To: Greg Thelen <gthelen@...gle.com>
Cc: Wang Long <wanglong19@...tuan.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Johannes Weiner <hannes@...xchg.org>,
Tejun Heo <tj@...nel.org>, npiggin@...il.com,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3] writeback: safer lock nesting
On Mon 09-04-18 17:59:08, Greg Thelen 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
>
Very nice changelog!
> 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.
Agreed!
> Reported-by: Wang Long <wanglong19@...tuan.com>
> Signed-off-by: Greg Thelen <gthelen@...gle.com>
> Change-Id: Ibb773e8045852978f6207074491d262f1b3fb613
Not a stable material IMHO but
Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
AFAIU
Acked-by: Michal Hocko <mhocko@...e.com>
Thanks!
> ---
> Changelog since v2:
> - explicitly initialize wb_lock_cookie to silence compiler warnings.
>
> Changelog since v1:
> - add wb_lock_cookie to record lock context.
>
> 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 --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 1280f915079b..f4b2f6625913 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -745,11 +745,12 @@ 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 bfe86b54f6c1..0bd432a4d7bd 100644
> --- a/include/linux/backing-dev-defs.h
> +++ b/include/linux/backing-dev-defs.h
> @@ -223,6 +223,11 @@ 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 3e4ce54d84ab..1d744c61d996 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -346,7 +346,7 @@ static inline struct bdi_writeback *inode_to_wb(const 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
> @@ -354,12 +354,11 @@ static inline struct bdi_writeback *inode_to_wb(const 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 *@...kdep
> - * 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 *@...kie 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 *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.
> @@ -382,12 +381,14 @@ unlocked_inode_to_wb_begin(struct inode *inode, bool *lockedp)
> /**
> * unlocked_inode_to_wb_end - end inode wb access transaction
> * @inode: target inode
> - * @locked: *@...kedp 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 *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 586f31261c83..bc38a2a7a597 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2501,13 +2501,13 @@ 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 = {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 *page)
> 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 *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 *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);
> --
> 2.17.0.484.g0c8726318c-goog
--
Michal Hocko
SUSE Labs
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