memcg: vmalloc: simplify MEMCG_VMALLOC updates

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.

[shakeel.butt@linux.dev: add comment]
Link: https://lkml.kernel.org/r/bmlkdbqgwboyqrnxyom7n52fjmo76ux77jhqw5odc6c6dfon3h@zdylwtmlywbt
Link: https://lkml.kernel.org/r/20250403053326.26860-1-shakeel.butt@linux.dev
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 2d75116..44b735a 100644 (file)
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3371,12 +3371,13 @@ void vfree(const void *addr)
 
        if (unlikely(vm->flags & VM_FLUSH_RESET_PERMS))
                vm_reset_perms(vm);
+       /* All pages of vm should be charged to same memcg, so use first one. */
+       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);
                /*
                 * High-order allocs for huge vmallocs are split, so
                 * can be freed as an array of order-0 allocations
@@ -3672,12 +3673,10 @@ 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);
-       }
+       /* All pages of vm should be charged to same memcg, so use first one. */
+       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