Re: [PATCH] mm: memcontrol: prevent starvation when writing memory.high

[Roman Gushchin <guro@fb.com>]

On Tue, Jan 12, 2021 at 11:30:11AM -0500, Johannes Weiner wrote:
> When a value is written to a cgroup's memory.high control file, the
> write() context first tries to reclaim the cgroup to size before
> putting the limit in place for the workload. Concurrent charges from
> the workload can keep such a write() looping in reclaim indefinitely.
> 
> In the past, a write to memory.high would first put the limit in place
> for the workload, then do targeted reclaim until the new limit has
> been met - similar to how we do it for memory.max. This wasn't prone
> to the described starvation issue. However, this sequence could cause
> excessive latencies in the workload, when allocating threads could be
> put into long penalty sleeps on the sudden memory.high overage created
> by the write(), before that had a chance to work it off.
> 
> Now that memory_high_write() performs reclaim before enforcing the new
> limit, reflect that the cgroup may well fail to converge due to
> concurrent workload activity. Bail out of the loop after a few tries.
> 
> Fixes: 536d3bf261a2 ("mm: memcontrol: avoid workload stalls when lowering memory.high")
> Cc: <stable@vger.kernel.org> # 5.8+
> Reported-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  mm/memcontrol.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 605f671203ef..63a8d47c1cd3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6275,7 +6275,6 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>  
>  	for (;;) {
>  		unsigned long nr_pages = page_counter_read(&memcg->memory);
> -		unsigned long reclaimed;
>  
>  		if (nr_pages <= high)
>  			break;
> @@ -6289,10 +6288,10 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>  			continue;
>  		}
>  
> -		reclaimed = try_to_free_mem_cgroup_pages(memcg, nr_pages - high,
> -							 GFP_KERNEL, true);
> +		try_to_free_mem_cgroup_pages(memcg, nr_pages - high,
> +					     GFP_KERNEL, true);
>  
> -		if (!reclaimed && !nr_retries--)
> +		if (!nr_retries--)

Shouldn't it be (!reclaimed || !nr_retries) instead?

If reclaimed == 0, it probably doesn't make much sense to retry.

Otherwise the patch looks good to me.

Thanks!