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

[Uladzislau Rezki <urezki@gmail.com>]

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?

--
Uladzislau Rezki