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

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Apr 04, 2025 at 10:44:05AM -0700, Shakeel Butt wrote:
> 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
> 
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Thank you!

--
Uladzislau Rezki