On Sun, Apr 22, 2018 at 5:24 AM Greg Kroah-Hartman < gregkh@linuxfoundation.org> wrote:
On Sun, Apr 22, 2018 at 04:15:12AM -0700, Nathan Chancellor wrote:
From: Greg Thelen gthelen@google.com
commit 2e898e4c0a3897ccd434adac5abb8330194f527b upstream.
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_memcgDue 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. Stable 4.4 has minor conflicts applying this patch. For a clean 4.4
patch
see "[PATCH for-4.4] writeback: safer lock nesting" https://lkml.org/lkml/2018/4/11/146
Wang Long said "this deadlock occurs three times in our environment"
[gthelen@google.com: v4] Link:
http://lkml.kernel.org/r/20180411084653.254724-1-gthelen@google.com
[akpm@linux-foundation.org: comment tweaks, struct initialization
simplification]
Change-Id: Ibb773e8045852978f6207074491d262f1b3fb613 Link:
http://lkml.kernel.org/r/20180410005908.167976-1-gthelen@google.com
Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb
transaction and use it for stat updates")
Signed-off-by: Greg Thelen gthelen@google.com Reported-by: Wang Long wanglong19@meituan.com Acked-by: Wang Long wanglong19@meituan.com Acked-by: Michal Hocko mhocko@suse.com Reviewed-by: Andrew Morton akpm@linux-foundation.org Cc: Johannes Weiner hannes@cmpxchg.org Cc: Tejun Heo tj@kernel.org Cc: Nicholas Piggin npiggin@gmail.com Cc: stable@vger.kernel.org [v4.2+] Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org [natechancellor: Adjust context due to lack of b93b016313b3b] Signed-off-by: Nathan Chancellor natechancellor@gmail.com
Thanks for all of these, now queued up.
greg k-h
I reviewed the 4.4, 4.9, 4.14, 4.16 queued stable backports for ("writeback: safer lock nesting"). They all look good. Thanks.
I don't know if it's customary to add an author's reviewed-by to non-trivial backports. If useful, here you go:
Reviewed-by: Greg Thelen gthelen@google.com