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Date: Mon, 9 Apr 2018 17:59:08 -0700
From: Greg Thelen <gthelen@...gle.com>
To: Wang Long <wanglong19@...tuan.com>,
Michal Hocko <mhocko@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>
Cc: npiggin@...il.com, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Greg Thelen <gthelen@...gle.com>
Subject: [PATCH v3] 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.
Reported-by: Wang Long <wanglong19@...tuan.com>
Signed-off-by: Greg Thelen <gthelen@...gle.com>
Change-Id: Ibb773e8045852978f6207074491d262f1b3fb613
---
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
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