The vm_area_alloc_pages() function uses cond_resched() to yield the
CPU during potentially long-running loops. However, these loops are
not considered long-running under normal conditions. In non-blocking
contexts, calling cond_resched() is inappropriate also.

Remove these calls to ensure correctness for blocking/non-blocking
contexts. This also simplifies the code path. In fact, a slow path
of page allocator already includes reschedule points to mitigate
latency.

This patch was tested for !CONFIG_PREEMPT kernel and with large
allocation chunks(~1GB), without triggering any "BUG: soft lockup"
warnings.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 mm/vmalloc.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 81b6d3bde719..b0255e0c74b3 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3633,7 +3633,6 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
 							pages + nr_allocated);
 
 			nr_allocated += nr;
-			cond_resched();
 
 			/*
 			 * If zero or pages were obtained partly,
@@ -3675,7 +3674,6 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
 		for (i = 0; i < (1U << order); i++)
 			pages[nr_allocated + i] = page + i;
 
-		cond_resched();
 		nr_allocated += 1U << order;
 	}
 
-- 
2.39.5

On 08/07/25 at 09:58am, Uladzislau Rezki (Sony) wrote:
> The vm_area_alloc_pages() function uses cond_resched() to yield the
> CPU during potentially long-running loops. However, these loops are
> not considered long-running under normal conditions. In non-blocking
> contexts, calling cond_resched() is inappropriate also.
> 
> Remove these calls to ensure correctness for blocking/non-blocking
> contexts. This also simplifies the code path. In fact, a slow path
> of page allocator already includes reschedule points to mitigate
> latency.
> 
> This patch was tested for !CONFIG_PREEMPT kernel and with large
> allocation chunks(~1GB), without triggering any "BUG: soft lockup"
> warnings.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
>  mm/vmalloc.c | 2 --
>  1 file changed, 2 deletions(-)

Reviewed-by: Baoquan He <bhe@redhat.com>

> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 81b6d3bde719..b0255e0c74b3 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3633,7 +3633,6 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
>  							pages + nr_allocated);
>  
>  			nr_allocated += nr;
> -			cond_resched();
>  
>  			/*
>  			 * If zero or pages were obtained partly,
> @@ -3675,7 +3674,6 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
>  		for (i = 0; i < (1U << order); i++)
>  			pages[nr_allocated + i] = page + i;
>  
> -		cond_resched();
>  		nr_allocated += 1U << order;
>  	}
>  
> -- 
> 2.39.5
>

On Thu 07-08-25 09:58:06, Uladzislau Rezki wrote:
> The vm_area_alloc_pages() function uses cond_resched() to yield the
> CPU during potentially long-running loops. However, these loops are
> not considered long-running under normal conditions.

To be more precise they can take long if they dive into the page
allocator but that already involves cond_rescheds where appropriate so
these are not needed in fact.

> In non-blocking
> contexts, calling cond_resched() is inappropriate also.
> 
> Remove these calls to ensure correctness for blocking/non-blocking
> contexts. This also simplifies the code path. In fact, a slow path
> of page allocator already includes reschedule points to mitigate
> latency.
> 
> This patch was tested for !CONFIG_PREEMPT kernel and with large
> allocation chunks(~1GB), without triggering any "BUG: soft lockup"
> warnings.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!
> ---
>  mm/vmalloc.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 81b6d3bde719..b0255e0c74b3 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3633,7 +3633,6 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
>  							pages + nr_allocated);
>  
>  			nr_allocated += nr;
> -			cond_resched();
>  
>  			/*
>  			 * If zero or pages were obtained partly,
> @@ -3675,7 +3674,6 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
>  		for (i = 0; i < (1U << order); i++)
>  			pages[nr_allocated + i] = page + i;
>  
> -		cond_resched();
>  		nr_allocated += 1U << order;
>  	}
>  
> -- 
> 2.39.5
> 

-- 
Michal Hocko
SUSE Labs

On Thu, Aug 07, 2025 at 01:22:36PM +0200, Michal Hocko wrote:
> On Thu 07-08-25 09:58:06, Uladzislau Rezki wrote:
> > The vm_area_alloc_pages() function uses cond_resched() to yield the
> > CPU during potentially long-running loops. However, these loops are
> > not considered long-running under normal conditions.
> 
> To be more precise they can take long if they dive into the page
> allocator but that already involves cond_rescheds where appropriate so
> these are not needed in fact.
> 
> > In non-blocking
> > contexts, calling cond_resched() is inappropriate also.
> > 
> > Remove these calls to ensure correctness for blocking/non-blocking
> > contexts. This also simplifies the code path. In fact, a slow path
> > of page allocator already includes reschedule points to mitigate
> > latency.
> > 
> > This patch was tested for !CONFIG_PREEMPT kernel and with large
> > allocation chunks(~1GB), without triggering any "BUG: soft lockup"
> > warnings.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Thanks!
>
Updated the commit message. Right, it can take long time.

Thank you!

--
Uladzislau Rezki