Re: [PATCH 2/2] mm/vmalloc: Add attempt_larger_order_alloc parameter

[Ryan Roberts <ryan.roberts@arm.com>]

On 16/12/2025 21:19, Uladzislau Rezki (Sony) wrote:
> Introduce a module parameter to enable or disable the large-order
> allocation path in vmalloc. High-order allocations are disabled by
> default so far, but users may explicitly enable them at runtime if
> desired.
> 
> High-order pages allocated for vmalloc are immediately split into
> order-0 pages and later freed as order-0, which means they do not
> feed the per-CPU page caches. As a result, high-order attempts tend
> to bypass the PCP fastpath and fall back to the buddy allocator that
> can affect performance.
> 
> However, when the PCP caches are empty, high-order allocations may
> show better performance characteristics especially for larger
> allocation requests.

I wonder if a better solution would be "allocate order-0 if available in pcp,
else try large order, else fallback to order-0" Could that provide the best of
all worlds without needing a configuration knob?

> 
> Since the best strategy is workload-dependent, this patch adds a
> parameter letting users to choose whether vmalloc should try
> high-order allocations or stay strictly on the order-0 fastpath.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
>  mm/vmalloc.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index d3a4725e15ca..f66543896b16 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -43,6 +43,7 @@
>  #include <asm/tlbflush.h>
>  #include <asm/shmparam.h>
>  #include <linux/page_owner.h>
> +#include <linux/moduleparam.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/vmalloc.h>
> @@ -3671,6 +3672,9 @@ vm_area_alloc_pages_large_order(gfp_t gfp, int nid, unsigned int order,
>  	return nr_allocated;
>  }
>  
> +static int attempt_larger_order_alloc;
> +module_param(attempt_larger_order_alloc, int, 0644);

Would this be better as a bool? Docs say that you can then specify 0/1, y/n or
Y/N as the value; that's probably more intuitive?

nit: I'd favour a shorter name. Perhaps large_order_alloc?

Thanks,
Ryan

> +
>  static inline unsigned int
>  vm_area_alloc_pages(gfp_t gfp, int nid,
>  		unsigned int order, unsigned int nr_pages, struct page **pages)
> @@ -3679,8 +3683,9 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
>  	struct page *page;
>  	int i;
>  
> -	nr_allocated = vm_area_alloc_pages_large_order(gfp, nid,
> -		order, nr_pages, pages);
> +	if (attempt_larger_order_alloc)
> +		nr_allocated = vm_area_alloc_pages_large_order(gfp, nid,
> +			order, nr_pages, pages);
>  
>  	/*
>  	 * For order-0 pages we make use of bulk allocator, if