Re: [PATCH v3 2/4] mm: add vmstat for cgroup uncharged pages

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

On Wed, Aug 20, 2025 at 12:46:43AM +0100, Matthew Wilcox wrote:
> On Tue, Aug 19, 2025 at 08:53:59AM -0700, Shakeel Butt wrote:
> > My initial thinking was based on Qu's original proposal which was using
> > root memcg where there will not be any difference between accounted
> > file pages and system wide file pages. However with Boris's change, we
> > can actually get the estimate, as you pointed out, by subtracting the
> > number of accounted file pages from system wide number of file pages.
> > 
> > However I still think we should keep this new metric because of
> > performance reason. To get accounted file pages, we need to read
> > memory.stat of the root memcg which can be very expensive. Basically it
> > may have to flush the rstat update trees on all the CPUs on the system.
> > Since this new metric will be used to calculate system overhead, the
> > high cost will limit how frequently a user can query the latest stat.
> 
> OK, but couldn't we make that argument for anything else?  Like slab,
> say.  Why's "file" memory different?

Good point and I think it does apply to other memory types too. I would
call "file" memory to be more important as it is one of the largest
consumer of DRAM on, at least, Meta infra. Slab needs a bit more thought.
At the system level (i.e. /proc/meminfo), we account at the page (or
slab) level while for memcg, we account per-object (plus obj_cgroup
pointer).