Re: [PATCH AUTOSEL 4.4 03/16] fs/btrfs: Add cond_resched() for try_release_extent_mapping() stalls

[David Sterba <dsterba@suse.cz>]

On Mon, Aug 10, 2020 at 03:14:30PM -0400, Sasha Levin wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> [ Upstream commit 9f47eb5461aaeb6cb8696f9d11503ae90e4d5cb0 ]
> 
> Very large I/Os can cause the following RCU CPU stall warning:
> 
> RIP: 0010:rb_prev+0x8/0x50
> Code: 49 89 c0 49 89 d1 48 89 c2 48 89 f8 e9 e5 fd ff ff 4c 89 48 10 c3 4c =
> 89 06 c3 4c 89 40 10 c3 0f 1f 00 48 8b 0f 48 39 cf 74 38 <48> 8b 47 10 48 85 c0 74 22 48 8b 50 08 48 85 d2 74 0c 48 89 d0 48
> RSP: 0018:ffffc9002212bab0 EFLAGS: 00000287 ORIG_RAX: ffffffffffffff13
> RAX: ffff888821f93630 RBX: ffff888821f93630 RCX: ffff888821f937e0
> RDX: 0000000000000000 RSI: 0000000000102000 RDI: ffff888821f93630
> RBP: 0000000000103000 R08: 000000000006c000 R09: 0000000000000238
> R10: 0000000000102fff R11: ffffc9002212bac8 R12: 0000000000000001
> R13: ffffffffffffffff R14: 0000000000102000 R15: ffff888821f937e0
>  __lookup_extent_mapping+0xa0/0x110
>  try_release_extent_mapping+0xdc/0x220
>  btrfs_releasepage+0x45/0x70
>  shrink_page_list+0xa39/0xb30
>  shrink_inactive_list+0x18f/0x3b0
>  shrink_lruvec+0x38e/0x6b0
>  shrink_node+0x14d/0x690
>  do_try_to_free_pages+0xc6/0x3e0
>  try_to_free_mem_cgroup_pages+0xe6/0x1e0
>  reclaim_high.constprop.73+0x87/0xc0
>  mem_cgroup_handle_over_high+0x66/0x150
>  exit_to_usermode_loop+0x82/0xd0
>  do_syscall_64+0xd4/0x100
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> On a PREEMPT=n kernel, the try_release_extent_mapping() function's
> "while" loop might run for a very long time on a large I/O.  This commit
> therefore adds a cond_resched() to this loop, providing RCU any needed
> quiescent states.
> 
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Paul,

this patch was well hidden in some huge RCU pile
(https://lore.kernel.org/lkml/20200623002147.25750-11-paulmck@kernel.org/)

I wonder why you haven't CCed linux-btrfs, I spotted the patch queued
for stable by incidentally. The timestamp is from June, that's quite
some time ago. We can deal with one more patch and I tend to reply with
acks quickly for easy patches like this to not block other peoples work
but I'm a bit disappointed by sidestepping maintained subsystems. It's
not just this patch, it happens from time time only to increase the
disapointement.