Re: [PATCH] mm: remove lock_page_memcg() from rmap

From: Shakeel Butt <shakeelb@google.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	linux-mm@kvack.org, cgroups@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: remove lock_page_memcg() from rmap
Date: Tue, 29 Nov 2022 11:42:54 -0800	[thread overview]
Message-ID: <CALvZod6PCmbA1cNVueuJL=njL+ZMez_rFK74GEmRZpNy_k=AUA@mail.gmail.com> (raw)
In-Reply-To: <Y4ZYsrXLBFDIxuoO@cmpxchg.org>

On Tue, Nov 29, 2022 at 11:08 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Mon, Nov 28, 2022 at 11:59:53AM -0500, Johannes Weiner wrote:
> > On Wed, Nov 23, 2022 at 10:03:00PM -0800, Hugh Dickins wrote:
> > The swapcache/pagecache bit was a brainfart. We acquire the folio lock
> > in move_account(), which would lock out concurrent faults. If it's not
> > mapped, I don't see how it could become mapped behind our backs. But
> > we do need to be prepared for it to be unmapped.
>
> Welp, that doesn't protect us from the inverse, where the page is
> mapped elsewhere and the other ptes are going away. So this won't be
> enough, unfortunately.
>
> > > Does that mean that we just have to reinstate the folio_mapped() checks
> > > in mm/memcontrol.c i.e. revert all mm/memcontrol.c changes from the
> > > commit?  Or does it invalidate the whole project to remove
> > > lock_page_memcg() from mm/rmap.c?
>
> Short of further restricting the pages that can be moved, I don't see
> how we can get rid of the cgroup locks in rmap after all. :(
>
> We can try limiting move candidates to present ptes. But maybe it's
> indeed time to deprecate the legacy charge moving altogether, and get
> rid of the entire complication.
>
> Hugh, Shakeel, Michal, what do you think?

I am on-board.