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

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Message-ID: <20201001205143.GE2706729@carbon.DHCP.thefacebook.com>
Date:   Thu, 1 Oct 2020 13:51:43 -0700
From:   Roman Gushchin <guro@...com>
To:     Johannes Weiner <hannes@...xchg.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <kernel-team@...com>
Subject: Re: [PATCH v4 1/4] mm: memcontrol: use helpers to access page's
 memcg data

On Thu, Oct 01, 2020 at 02:59:50PM -0400, Johannes Weiner wrote:
> 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.

I'm not ignoring: I thought you was looking to remove clear_page_* functions,
but it wasn't clear you want eliminate set_page_memcg() function. Please, when
you asking for some "style" changes, please, provide some rationale, it's way
less obvious than you think, what exactly you don't like in the proposed version.

Thanks.