Re: [PATCH v4 1/4] mm: memcontrol: use helpers to access page's memcg data

[Johannes Weiner]

On Thu, Oct 01, 2020 at 11:27:39AM -0700, Roman Gushchin wrote:
> On Thu, Oct 01, 2020 at 09:46:38AM -0400, Johannes Weiner wrote:
> > On Wed, Sep 30, 2020 at 05:27:07PM -0700, Roman Gushchin wrote:
> > > +/*
> > > + * set_page_memcg - associate a page with a memory cgroup
> > > + * @page: a pointer to the page struct
> > > + * @memcg: a pointer to the memory cgroup
> > > + *
> > > + * Associates a page with a memory cgroup.
> > > + */
> > > +static inline void set_page_memcg(struct page *page, struct mem_cgroup *memcg)
> > > +{
> > > +	VM_BUG_ON_PAGE(PageSlab(page), page);
> > > +
> > > +	/*
> > > +	 * Please, refer to page_memcg()'s description for the page and memcg
> > > +	 * binding stability requirements.
> > > +	 */
> > > +	page->memcg_data = (unsigned long)memcg;
> > > +}
> > 
> > Please delete and inline this as per previous feedback, thanks.
> 
> Why it's better?
> It's ok for set_page_memcg(), but obviously worse for set_page_objcgs():
> it was nice to have all bit magic in one place, in few helper functions.
> And now it spills into several places. What's the win?

set_page_objcgs() is a worthwhile abstraction because it includes the
synchronization primitives that make it safe to use wrt
page_objcgs(). They encapsulate the cmpxchg and the READ_ONCE().

set_page_memcg() doesn't do any synchronization and relies fully on
the contextual locking. The name implies that it includes things to
make it safe wrt page_memcg(), which isn't true at all. It's a long
and misleading name for '='.

Btw, I really don't mind having this discussion, but please don't send
revisions that silently ignore feedback you don't agree with.

Thanks