Re: [PATCH AUTOSEL 6.0 64/67] sbitmap: fix lockup while swapping

[Hugh Dickins <hughd@google.com>]

On Wed, 12 Oct 2022, Sasha Levin wrote:

> From: Hugh Dickins <hughd@google.com>
> 
> [ Upstream commit 30514bd2dd4e86a3ecfd6a93a3eadf7b9ea164a0 ]
> 
> Commit 4acb83417cad ("sbitmap: fix batched wait_cnt accounting")
> is a big improvement: without it, I had to revert to before commit
> 040b83fcecfb ("sbitmap: fix possible io hung due to lost wakeup")
> to avoid the high system time and freezes which that had introduced.
> 
> Now okay on the NVME laptop, but 4acb83417cad is a disaster for heavy
> swapping (kernel builds in low memory) on another: soon locking up in
> sbitmap_queue_wake_up() (into which __sbq_wake_up() is inlined), cycling
> around with waitqueue_active() but wait_cnt 0 .  Here is a backtrace,
> showing the common pattern of outer sbitmap_queue_wake_up() interrupted
> before setting wait_cnt 0 back to wake_batch (in some cases other CPUs
> are idle, in other cases they're spinning for a lock in dd_bio_merge()):
> 
> sbitmap_queue_wake_up < sbitmap_queue_clear < blk_mq_put_tag <
> __blk_mq_free_request < blk_mq_free_request < __blk_mq_end_request <
> scsi_end_request < scsi_io_completion < scsi_finish_command <
> scsi_complete < blk_complete_reqs < blk_done_softirq < __do_softirq <
> __irq_exit_rcu < irq_exit_rcu < common_interrupt < asm_common_interrupt <
> _raw_spin_unlock_irqrestore < __wake_up_common_lock < __wake_up <
> sbitmap_queue_wake_up < sbitmap_queue_clear < blk_mq_put_tag <
> __blk_mq_free_request < blk_mq_free_request < dd_bio_merge <
> blk_mq_sched_bio_merge < blk_mq_attempt_bio_merge < blk_mq_submit_bio <
> __submit_bio < submit_bio_noacct_nocheck < submit_bio_noacct <
> submit_bio < __swap_writepage < swap_writepage < pageout <
> shrink_folio_list < evict_folios < lru_gen_shrink_lruvec <
> shrink_lruvec < shrink_node < do_try_to_free_pages < try_to_free_pages <
> __alloc_pages_slowpath < __alloc_pages < folio_alloc < vma_alloc_folio <
> do_anonymous_page < __handle_mm_fault < handle_mm_fault <
> do_user_addr_fault < exc_page_fault < asm_exc_page_fault
> 
> See how the process-context sbitmap_queue_wake_up() has been interrupted,
> after bringing wait_cnt down to 0 (and in this example, after doing its
> wakeups), before advancing wake_index and refilling wake_cnt: an
> interrupt-context sbitmap_queue_wake_up() of the same sbq gets stuck.
> 
> I have almost no grasp of all the possible sbitmap races, and their
> consequences: but __sbq_wake_up() can do nothing useful while wait_cnt 0,
> so it is better if sbq_wake_ptr() skips on to the next ws in that case:
> which fixes the lockup and shows no adverse consequence for me.
> 
> The check for wait_cnt being 0 is obviously racy, and ultimately can lead
> to lost wakeups: for example, when there is only a single waitqueue with
> waiters.  However, lost wakeups are unlikely to matter in these cases,
> and a proper fix requires redesign (and benchmarking) of the batched
> wakeup code: so let's plug the hole with this bandaid for now.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Reviewed-by: Jan Kara <jack@suse.cz>
> Reviewed-by: Keith Busch <kbusch@kernel.org>
> Link: https://lore.kernel.org/r/9c2038a7-cdc5-5ee-854c-fbc6168bf16@google.com
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> Signed-off-by: Sasha Levin <sashal@kernel.org>

Whoa!  NAK to this 6.0 backport, and to the 5.19, 5.15, 5.10, 5.4
AUTOSEL backports of the same commit.  I never experienced such a
lockup on those releases.  Or have I missed announcements of stable
backports of the whole series of 6.1-rc commits to which this one
is a fix?  (I hope not.)

I'm happy for my NAK to be overruled by Jens or Jan or Keith,
if they see virtue in this commit, beyond what I'm aware of:
but as it stands, it looks like AUTOSEL out of control again -
it found the word "fix", and found that the commit applies cleanly,
so thinks it must be a good stable addition.  Not necessarily so!

Hugh

> ---
>  lib/sbitmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 29eb0484215a..e000aaf6dbde 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -588,7 +588,7 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
>  	for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
>  		struct sbq_wait_state *ws = &sbq->ws[wake_index];
>  
> -		if (waitqueue_active(&ws->wait)) {
> +		if (waitqueue_active(&ws->wait) && atomic_read(&ws->wait_cnt)) {
>  			if (wake_index != atomic_read(&sbq->wake_index))
>  				atomic_set(&sbq->wake_index, wake_index);
>  			return ws;
> -- 
> 2.35.1