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

[Vlastimil Babka (SUSE)]

On 3/30/26 05:17, Andrew Morton wrote:
> On Sun, 29 Mar 2026 18:08:52 -0700 (PDT) David Rientjes <rientjes@xxxxxxxxxx> 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.
>> 
>> ...
>>
>> +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)) {
>> +		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));
> 
> Snould we use dump_page() in here?  It prints more info, does the
> snapshotting thing.

But we have no page to dump, or did you mean something else? Maybe some part
of warn_alloc() (without its own ratelimit etc) could be extracted end reused.

>> +		cpuset_print_current_mems_allowed();
>> +		pr_cont("\n");
>> +		dump_stack();
>> +		warn_alloc_show_mem(gfp_mask, nodemask);
>> +		return;
>> +	}
>> +
>> +	spin_unlock(&alloc_stall_lock);
>> +}
>> +
>