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

On Mon, Mar 22, 2021 at 11:36:19PM +0100, Uladzislau Rezki wrote:

On Mon, Mar 22, 2021 at 07:38:20PM +0000, Matthew Wilcox (Oracle) wrote:

quoted

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@infradead.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

Well, 4.5% isn't something to leave on the table ... but yeah, I was
expecting more in the 10-20% range.  It may be more significant if
there's contention on the spinlocks (like if this crazy ksmbd is calling
vmalloc(4MB) on multiple nodes simultaneously).

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?

https://lore.kernel.org/linux-mm/20210322091845.16437-1-mgorman@techsingularity.net/ (local)

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

We're pretty good about allocating memory in larger chunks these days.
Looking at my laptop's slabinfo,
kmalloc-8k           219    232   8192    4    8 : tunables    0    0    0 : sla
bdata     58     58      0

That's using 8 pages per slab, so that's order-3 allocations.  There's a
few more of those:

$ sudo grep '8 :' /proc/slabinfo |wc
     42     672    4508

so I have confidence that kvmalloc() will manage to use kmalloc up to 16MB
vmalloc allocations, and after that it'll tend to fall back to vmalloc.