Re: [PATCH] memcg: vmalloc: simplify MEMCG_VMALLOC updates

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Wed, Apr 02, 2025 at 10:33:26PM -0700, Shakeel Butt wrote:
> The vmalloc region can either be charged to a single memcg or none. At
> the moment kernel traverses all the pages backing the vmalloc region to
> update the MEMCG_VMALLOC stat. However there is no need to look at all
> the pages as all those pages will be charged to a single memcg or none.
> Simplify the MEMCG_VMALLOC update by just looking at the first page of
> the vmalloc region.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> ---
>  mm/vmalloc.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 3ed720a787ec..cdae76994488 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3370,12 +3370,12 @@ void vfree(const void *addr)
>  
>  	if (unlikely(vm->flags & VM_FLUSH_RESET_PERMS))
>  		vm_reset_perms(vm);
> +	if (vm->nr_pages && !(vm->flags & VM_MAP_PUT_PAGES))
> +		mod_memcg_page_state(vm->pages[0], MEMCG_VMALLOC, -vm->nr_pages);
>  	for (i = 0; i < vm->nr_pages; i++) {
>  		struct page *page = vm->pages[i];
>  
>  		BUG_ON(!page);
> -		if (!(vm->flags & VM_MAP_PUT_PAGES))
> -			mod_memcg_page_state(page, MEMCG_VMALLOC, -1);

We can add a debug check here (and/or in the vmalloc path) to check that
all pages are indeed charged to the same memcg.

Regardless, this change makes sense:
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>


>  		/*
>  		 * High-order allocs for huge vmallocs are split, so
>  		 * can be freed as an array of order-0 allocations
> @@ -3671,12 +3671,9 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  		node, page_order, nr_small_pages, area->pages);
>  
>  	atomic_long_add(area->nr_pages, &nr_vmalloc_pages);
> -	if (gfp_mask & __GFP_ACCOUNT) {
> -		int i;
> -
> -		for (i = 0; i < area->nr_pages; i++)
> -			mod_memcg_page_state(area->pages[i], MEMCG_VMALLOC, 1);
> -	}
> +	if (gfp_mask & __GFP_ACCOUNT && area->nr_pages)
> +		mod_memcg_page_state(area->pages[0], MEMCG_VMALLOC,
> +				     area->nr_pages);
>  
>  	/*
>  	 * If not enough pages were obtained to accomplish an
> -- 
> 2.47.1
> 
>