linux-kernel - Re: [PATCH 2/2] mm/vmalloc: Use kvmalloc to allocate the table of pages

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Message-ID: <20210322223619.GA56503@pc638.lan>
Date:   Mon, 22 Mar 2021 23:36:19 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Uladzislau Rezki <urezki@...il.com>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 2/2] mm/vmalloc: Use kvmalloc to allocate the table of
 pages

On Mon, Mar 22, 2021 at 07:38:20PM +0000, Matthew Wilcox (Oracle) wrote:
> If we're trying to allocate 4MB of memory, the table will be 8KiB in size
> (1024 pointers * 8 bytes per pointer), which can usually be satisfied
> by a kmalloc (which is significantly faster).  Instead of changing this
> open-coded implementation, just use kvmalloc().
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  mm/vmalloc.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 96444d64129a..32b640a84250 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2802,13 +2802,8 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  		gfp_mask |= __GFP_HIGHMEM;
>  
>  	/* Please note that the recursion is strictly bounded. */
> -	if (array_size > PAGE_SIZE) {
> -		pages = __vmalloc_node(array_size, 1, nested_gfp, node,
> +	pages = kvmalloc_node_caller(array_size, nested_gfp, node,
>  					area->caller);
> -	} else {
> -		pages = kmalloc_node(array_size, nested_gfp, node);
> -	}
> -
>  	if (!pages) {
>  		free_vm_area(area);
>  		return NULL;
> -- 
> 2.30.2
Makes sense to me. Though i expected a bigger difference:

# patch
single CPU, 4MB allocation, loops: 1000000 avg: 85293854 usec

# default
single CPU, 4MB allocation, loops: 1000000 avg: 89275857 usec

One question. Should we care much about fragmentation? I mean
with the patch, allocations > 2MB will do request to SLAB bigger
then PAGE_SIZE.

Thanks!

--
Vlad Rezki