Re: [patch] mm, page_alloc: reintroduce page allocation stall warning

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 3/30/26 03:08, David Rientjes wrote:
> Previously, we had warnings when a single page allocation took longer
> than reasonably expected.  This was introduced in commit 63f53dea0c98
> ("mm: warn about allocations which stall for too long").
> 
> The warning was subsequently reverted in commit 400e22499dd9 ("mm: don't
> warn about allocations which stall for too long") but for reasons
> unrelated to the warning itself.
> 
> Page allocation stalls in excess of 10 seconds are always useful to debug
> because they can result in severe userspace unresponsiveness.  Adding
> this artifact can be used to correlate with userspace going out to lunch
> and to understand the state of memory at the time.
> 
> There should be a reasonable expectation that this warning will never
> trigger given it is very passive, it will only be emitted when a page
> allocation takes longer than 10 seconds.  If it does trigger, this
> reveals an issue that should be fixed: a single page allocation should
> never loop for more than 10 seconds without oom killing to make memory
> available.
> 
> Unlike the original implementation, this implementation only reports
> stalls once for the system every 10 seconds.  Otherwise, many concurrent
> reclaimers could spam the kernel log unnecessarily.  Stalls are only
> reported when calling into direct reclaim.
> 
> Signed-off-by: David Rientjes <rientjes@google.com>

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

Nit below:

> ---
>  mm/page_alloc.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 46 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -316,6 +316,14 @@ EXPORT_SYMBOL(nr_node_ids);
>  EXPORT_SYMBOL(nr_online_nodes);
>  #endif
>  
> +/*
> + * When page allocations stall for longer than a threshold,
> + * ALLOC_STALL_WARN_MSECS, leave a warning in the kernel log.  Only one warning
> + * will be printed during this duration for the entire system.
> + */
> +#define ALLOC_STALL_WARN_MSECS (10 * 1000UL)
> +static unsigned long alloc_stall_warn_jiffies;
> +
>  static bool page_contains_unaccepted(struct page *page, unsigned int order);
>  static bool cond_accept_memory(struct zone *zone, unsigned int order,
>  			       int alloc_flags);
> @@ -4706,6 +4714,40 @@ check_retry_cpuset(int cpuset_mems_cookie, struct alloc_context *ac)
>  	return false;
>  }
>  
> +static void check_alloc_stall_warn(gfp_t gfp_mask, nodemask_t *nodemask,
> +				unsigned int order, unsigned long alloc_start_time)
> +{
> +	static DEFINE_SPINLOCK(alloc_stall_lock);
> +	unsigned long stall_msecs = jiffies_to_msecs(jiffies - alloc_start_time);
> +
> +	if (likely(stall_msecs < ALLOC_STALL_WARN_MSECS))
> +		return;
> +	if (time_before(jiffies, READ_ONCE(alloc_stall_warn_jiffies)))
> +		return;
> +	if (gfp_mask & __GFP_NOWARN)
> +		return;
> +
> +	if (!spin_trylock(&alloc_stall_lock))
> +		return;
> +
> +	if (time_after_eq(jiffies, alloc_stall_warn_jiffies)) {

This could also be an unlock+return if the opposite (time_before()) is true,
reducing the indentation for the actual warning code.

> +		WRITE_ONCE(alloc_stall_warn_jiffies,
> +			   jiffies + msecs_to_jiffies(ALLOC_STALL_WARN_MSECS));
> +		spin_unlock(&alloc_stall_lock);
> +
> +		pr_warn("%s: page allocation stall for %lu secs: order:%d, mode:%#x(%pGg) nodemask=%*pbl",
> +			current->comm, stall_msecs / MSEC_PER_SEC, order, gfp_mask, &gfp_mask,
> +			nodemask_pr_args(nodemask));
> +		cpuset_print_current_mems_allowed();
> +		pr_cont("\n");
> +		dump_stack();
> +		warn_alloc_show_mem(gfp_mask, nodemask);
> +		return;
> +	}
> +
> +	spin_unlock(&alloc_stall_lock);
> +}
> +