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

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Date:   Tue, 23 Mar 2021 14:39:48 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        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, Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 2/2] mm/vmalloc: Use kvmalloc to allocate the table of
 pages

On Tue, Mar 23, 2021 at 12:39:13PM +0000, Matthew Wilcox wrote:
> On Tue, Mar 23, 2021 at 01:04:36PM +0100, Uladzislau Rezki wrote:
> > On Mon, Mar 22, 2021 at 11:03:11PM +0000, Matthew Wilcox wrote:
> > > I suspect the vast majority of the time is spent calling alloc_pages_node()
> > > 1024 times.  Have you looked at Mel's patch to do ... well, exactly what
> > > vmalloc() wants?
> > > 
> > <snip>
> >          - __vmalloc_node_range
> >             - 45.25% __alloc_pages_nodemask
> >                - 37.59% get_page_from_freelist
> [...]
> >       - 44.61% 0xffffffffc047348d
> >          - __vunmap
> >             - 35.56% free_unref_page
> 
> Hmm!  I hadn't been thinking about the free side of things.
> Does this make a difference?
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 4f5f8c907897..61d5b769fea0 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2277,16 +2277,8 @@ static void __vunmap(const void *addr, int deallocate_pages)
>  	vm_remove_mappings(area, deallocate_pages);
>  
>  	if (deallocate_pages) {
> -		int i;
> -
> -		for (i = 0; i < area->nr_pages; i++) {
> -			struct page *page = area->pages[i];
> -
> -			BUG_ON(!page);
> -			__free_pages(page, 0);
> -		}
> +		release_pages(area->pages, area->nr_pages);
>  		atomic_long_sub(area->nr_pages, &nr_vmalloc_pages);
> -
>  		kvfree(area->pages);
>  	}
> 
Will check it today!

> release_pages does a bunch of checks that are unnecessary ... we could
> probably just do:
> 
> 		LIST_HEAD(pages_to_free);
> 
> 		for (i = 0; i < area->nr_pages; i++) {
> 			struct page *page = area->pages[i];
> 			if (put_page_testzero(page))
> 				list_add(&page->lru, &pages_to_free);
> 		}
> 		free_unref_page_list(&pages_to_free);
> 
> but let's see if the provided interface gets us the performance we want.
>  
> > Reviewed-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > 
> > Thanks!
> 
> Thank you!
You are welcome. A small nit:

  CC      mm/vmalloc.o
mm/vmalloc.c: In function ‘__vmalloc_area_node’:
mm/vmalloc.c:2492:14: warning: passing argument 4 of ‘kvmalloc_node_caller’ makes integer from pointer without a cast [-Wint-conversion]
          area->caller);
          ~~~~^~~~~~~~
In file included from mm/vmalloc.c:12:
./include/linux/mm.h:782:7: note: expected ‘long unsigned int’ but argument is of type ‘const void *’
 void *kvmalloc_node_caller(size_t size, gfp_t flags, int node,

<snip>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 8a202ba263f6..ee6fa44983bc 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2489,7 +2489,7 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 
        /* Please note that the recursion is strictly bounded. */
        pages = kvmalloc_node_caller(array_size, nested_gfp, node,
-                                                                area->caller);
+                               (unsigned long) area->caller);
        if (!pages) {
                free_vm_area(area);
                return NULL;
<snip>

As for the bulk-array interface. I have checked the:

git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-bulk-rebase-v6r2

applied the patch that is in question + below one:

<snip>
@@ -2503,25 +2498,13 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
        area->pages = pages;
        area->nr_pages = nr_pages;
 
-       for (i = 0; i < area->nr_pages; i++) {
-               struct page *page;
-
-               if (node == NUMA_NO_NODE)
-                       page = alloc_page(gfp_mask);
-               else
-                       page = alloc_pages_node(node, gfp_mask, 0);
-
-               if (unlikely(!page)) {
-                       /* Successfully allocated i pages, free them in __vfree() */
-                       area->nr_pages = i;
-                       atomic_long_add(area->nr_pages, &nr_vmalloc_pages);
-                       goto fail;
-               }
-               area->pages[i] = page;
-               if (gfpflags_allow_blocking(gfp_mask))
-                       cond_resched();
+       ret = alloc_pages_bulk_array(gfp_mask, area->nr_pages, area->pages);
+       if (ret == nr_pages)
+               atomic_long_add(area->nr_pages, &nr_vmalloc_pages);
+       else {
+               area->nr_pages = ret;
+               goto fail;
        }
-       atomic_long_add(area->nr_pages, &nr_vmalloc_pages);
<snip>

single CPU, 4MB allocation, 1000000 avg: 70639437 usec
single CPU, 4MB allocation, 1000000 avg: 89218654 usec

and now we get ~21% delta. That is very good :)

--
Vlad Rezki