Re: [PATCH] mm: memcontrol: remove page_memcg()

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

On Thu, May 23, 2024 at 02:31:05PM +0100, Matthew Wilcox wrote:
> On Tue, May 21, 2024 at 12:29:39PM -0700, Shakeel Butt wrote:
> > On Tue, May 21, 2024 at 03:44:21PM +0100, Matthew Wilcox wrote:
> > > The memcg should not be attached to the individual pages that make up a
> > > vmalloc allocation.  Rather, it should be managed by the vmalloc
> > > allocation itself.  I don't have the knowledge to poke around inside
> > > vmalloc right now, but maybe somebody else could take that on.
> > 
> > Are you concerned about accessing just memcg or any field of the
> > sub-page? There are drivers accessing fields of pages allocated through
> > vmalloc. Some details at 3b8000ae185c ("mm/vmalloc: huge vmalloc backing
> > pages should be split rather than compound").
> 
> Thanks for the pointer, and fb_deferred_io_fault() is already on my
> hitlist for abusing struct page.
> 
> My primary concern is that we should track the entire allocation as a
> single object rather than tracking each page individually.  That means
> assigning the vmalloc allocation to a memcg rather than assigning each
> page to a memcg.  It's a lot less overhead to increment the counter once
> per allocation rather than once per page in the allocation!
> 
> But secondarily, yes, pages allocated by vmalloc probably don't need
> any per-page state, other than tracking the vmalloc allocation they're
> assigned to.  We'll see how that theory turns out.

I think the tricky part would be vmalloc having pages spanning multiple
nodes which is not an issue for MEMCG_VMALLOC stat but the vmap based
kernel stack (CONFIG_VMAP_STACK) metric NR_KERNEL_STACK_KB cares about
that information.