Re: [PATCH] mm: memcg: remove direct use of __memcg_kmem_uncharge_page

[Shakeel Butt <shakeelb@google.com>]

On Wed, Dec 13, 2023 at 12:27 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Dec 13, 2023 at 08:23:25PM +0000, Shakeel Butt wrote:
> > On Wed, Dec 13, 2023 at 08:31:00AM -0800, Yosry Ahmed wrote:
> > > On Wed, Dec 13, 2023 at 8:27 AM Matthew Wilcox <willy@infradead.org> wrote:
> > > >
> > > > On Wed, Dec 13, 2023 at 08:24:04AM -0800, Yosry Ahmed wrote:
> > > > > I doubt an extra compound_head() will matter in that path, but if you
> > > > > feel strongly about it that's okay. It's a nice cleanup that's all.
> > > >
> > > > i don't even understand why you think it's a nice cleanup.
> > >
> > > free_pages_prepare() is directly calling __memcg_kmem_uncharge_page()
> > > instead of memcg_kmem_uncharge_page(), and open-coding checks that
> > > already exist in both of them to avoid the unnecessary function call
> > > if possible. I think this should be the job of
> > > memcg_kmem_uncharge_page(), but it's currently missing the
> > > PageMemcgKmem() check (which is in __memcg_kmem_uncharge_page()).
> > >
> > > So I think moving that check to the wrapper allows
> > > free_pages_prepare() to call memcg_kmem_uncharge_page() and without
> > > worrying about those memcg-specific checks.
> >
> > There is a (performance) reason these open coded check are present in
> > page_alloc.c and that is very clear for __memcg_kmem_charge_page() but
> > not so much for __memcg_kmem_uncharge_page(). So, for uncharge path,
> > this seems ok. Now to resolve Willy's concern for the fork() path, I
> > think we can open code the checks there.
> >
> > Willy, any concern with that approach?
>
> The justification for this change is insufficient.  Or really any change
> in this area.  It's fine the way it is.  "The check is done twice" is
> really weak, when the check is so cheap (much cheaper than calling
> compound_head!)

I think that is what Yosry is trying i.e. reducing two calls to
page_folio() to one in the page free path.