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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Apr 04, 2025 at 12:34:33PM +0200, Uladzislau Rezki wrote:
> On Thu, Apr 03, 2025 at 11:20:18AM -0700, Shakeel Butt wrote:
> > On Thu, Apr 03, 2025 at 01:17:22PM +0200, Uladzislau Rezki wrote:
> > > 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);
> > > >
> > > Could you please add a comment stating that the first page should be
> > > modified?
> > > 
> > 
> > Sorry, what do you mean by first page should be modified?
> > mod_memcg_page_state() will not modify the page but extract memcg from
> > it and modify its vmalloc stat.
> > 
> I meant what you wrote in the commit message. A mod_memcg_page_state() can
> be invoked only on a first page within a mapped range, because the rest is
> anyway is associated with the same mem_cgroup struct.
> 
> Just add a comment that we do not need to check all pages. Can you add it?

Ack. Andrew, please squash the following into the patch.


From 982971062e6bd04feabf4f6a745469cb9bddef03 Mon Sep 17 00:00:00 2001
From: Shakeel Butt <shakeel.butt@linux.dev>
Date: Fri, 4 Apr 2025 10:41:52 -0700
Subject: [PATCH] memcg : simplify MEMCG_VMALLOC updates - fix

Add comment

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 mm/vmalloc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index cdae76994488..bcc90d4357e4 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3370,6 +3370,7 @@ 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++) {
@@ -3671,6 +3672,7 @@ 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);
+	/* 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);
-- 
2.47.1