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

Michal Hocko
2026-03-23T16:53:58Z

On Sat 21-03-26 20:03:16, 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 starts with a 10 second floor to
> begin with. 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 that are at least a second longer than the longest stall reported
> thus far.

Am all for reintroducing the warning in some shape. The biggest problem
back then was printk being too eager to stomp all the work at a single
executing context. Not sure this is still the case. Let's add printk
maintainers.
Also it makes some sense to differentiate stalled callers and show_mem
which is more verbose. The former tells us who is affected and the
second will give us more context and we want to get some information
about all of them. The latter can be printed much less often as it will
describe situation for a batch of concurrent ones.

> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
> mm/page_alloc.c | 32 ++++++++++++++++++++++++++++++++
> 1 file changed, 32 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4706,6 +4706,36 @@ check_retry_cpuset(int cpuset_mems_cookie, struct alloc_context *ac)
> return false;
> }
>
> +static unsigned long max_alloc_stall_warn_msecs = 10 * 1000L;
> +
> +static void check_alloc_stall_warn(gfp_t gfp_mask, nodemask_t *nodemask,
> + unsigned int order, unsigned long alloc_start_time)
> +{
> + static DEFINE_SPINLOCK(max_alloc_stall_lock);
> + unsigned long stall_msecs = jiffies_to_msecs(jiffies - alloc_start_time);
> + unsigned long flags;
> +
> + if (likely(stall_msecs <= READ_ONCE(max_alloc_stall_warn_msecs)))
> + return;
> + if (gfp_mask & __GFP_NOWARN)
> + return;
> +
> + spin_lock_irqsave(&max_alloc_stall_lock, flags);
> + if (stall_msecs > max_alloc_stall_warn_msecs) {
> + 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);
> +
> + /* Only print future stalls that are more than a second longer */
> + WRITE_ONCE(max_alloc_stall_warn_msecs, stall_msecs + MSEC_PER_SEC);
> + }
> + spin_unlock_irqrestore(&max_alloc_stall_lock, flags);
> +}
> +
> static inline struct page *
> __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> struct alloc_context *ac)
> @@ -4726,6 +4756,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> int reserve_flags;
> bool compact_first = false;
> bool can_retry_reserves = true;
> + unsigned long alloc_start_time = jiffies;
>
> if (unlikely(nofail)) {
> /*
> @@ -4990,6 +5021,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> warn_alloc(gfp_mask, ac->nodemask,
> "page allocation failure: order:%u", order);
> got_pg:
> + check_alloc_stall_warn(gfp_mask, ac->nodemask, order, alloc_start_time);
> return page;
> }
>

--
Michal Hocko
SUSE Labs