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Date: Tue, 10 Apr 2018 13:37:59 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Greg Thelen <gthelen@...gle.com>
Cc: Wang Long <wanglong19@...tuan.com>,
Michal Hocko <mhocko@...nel.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, 9 Apr 2018 17:59:08 -0700 Greg Thelen <gthelen@...gle.com> 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
>
> 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.
>
> ...
>
> Changelog since v2:
> - explicitly initialize wb_lock_cookie to silence compiler warnings.
But only in some places. What's up with that?
>
> ...
>
> --- 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.
> */
Grammar is a bit awkward here,
>
> ...
>
> --- 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};
Trivia: it's better to use "= {}" here. That has the same effect and
it doesn't assume that the first field is a scalar. And indeed, the
first field is a bool so it should be {false}!
So...
--- a/include/linux/backing-dev.h~writeback-safer-lock-nesting-fix
+++ a/include/linux/backing-dev.h
@@ -355,7 +355,8 @@ static inline struct bdi_writeback *inod
* unlocked_inode_to_wb_end().
*
* 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.
+ * 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, struct wb_lock_cookie *cookie)
--- a/mm/page-writeback.c~writeback-safer-lock-nesting-fix
+++ a/mm/page-writeback.c
@@ -2501,7 +2501,7 @@ void account_page_redirty(struct page *p
if (mapping && mapping_cap_account_dirty(mapping)) {
struct inode *inode = mapping->host;
struct bdi_writeback *wb;
- struct wb_lock_cookie cookie = {0};
+ struct wb_lock_cookie cookie = {};
wb = unlocked_inode_to_wb_begin(inode, &cookie);
current->nr_dirtied--;
@@ -2613,7 +2613,7 @@ void __cancel_dirty_page(struct page *pa
if (mapping_cap_account_dirty(mapping)) {
struct inode *inode = mapping->host;
struct bdi_writeback *wb;
- struct wb_lock_cookie cookie = {0};
+ struct wb_lock_cookie cookie = {};
lock_page_memcg(page);
wb = unlocked_inode_to_wb_begin(inode, &cookie);
@@ -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;
- struct wb_lock_cookie cookie = {0};
+ struct wb_lock_cookie cookie = {};
/*
* Yes, Virginia, this is indeed insane.
But I wonder about the remaining uninitialized wb_lock_cookies?
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