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Subject[PATCH for-4.4] writeback: safer lock nesting
Date
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.

[ 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
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