linux-kernel - Re: [PATCH v2] mm: page-writeback: simplify memcg handling in test_clear_page

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Date:   Wed, 10 Feb 2021 14:59:32 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Roman Gushchin <guro@...com>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>,
        Arjun Roy <arjunroy@...gle.com>
Subject: Re: [PATCH v2] mm: page-writeback: simplify memcg handling in test_clear_page_writeback()

On Wed, Feb 10, 2021 at 9:44 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Wed, Feb 10, 2021 at 08:22:00AM -0800, Hugh Dickins wrote:
> > On Tue, 9 Feb 2021, Hugh Dickins wrote:
> > > On Tue, 9 Feb 2021, Johannes Weiner wrote:
> > >
> > > > Page writeback doesn't hold a page reference, which allows truncate to
> > > > free a page the second PageWriteback is cleared. This used to require
> > > > special attention in test_clear_page_writeback(), where we had to be
> > > > careful not to rely on the unstable page->memcg binding and look up
> > > > all the necessary information before clearing the writeback flag.
> > > >
> > > > Since commit 073861ed77b6 ("mm: fix VM_BUG_ON(PageTail) and
> > > > BUG_ON(PageWriteback)") test_clear_page_writeback() is called with an
> > > > explicit reference on the page, and this dance is no longer needed.
> > > >
> > > > Use unlock_page_memcg() and dec_lruvec_page_stat() directly.
> > >
> > > s/stat()/state()/
> > >
> > > This is a nice cleanup: I hadn't seen that connection at all.
> > >
> > > But I think you should take it further:
> > > __unlock_page_memcg() can then be static in mm/memcontrol.c,
> > > and its declarations deleted from include/linux/memcontrol.h?
> >
> > And further: void lock_page_memcg(page), not returning memcg.
>
> You're right on all counts!
>
> > > And further: delete __dec_lruvec_state() and dec_lruvec_state()
> > > from include/linux/vmstat.h - unless you feel that every "inc"
> > > ought to be matched by a "dec", even when unused.
>
> Hey look, there isn't a user for the __inc, either :) There is one for
> inc, but I don't insist on having symmetry there.
>
> > > > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > >
> > > Acked-by: Hugh Dickins <hughd@...gle.com>
>
> Thanks for the review and good feedback.
>
> How about this v2?
>
> ---
>
> From 5bcc0f468460aa2670c40318bb657e8b08ef96d5 Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@...xchg.org>
> Date: Tue, 9 Feb 2021 16:22:42 -0500
> Subject: [PATCH] mm: page-writeback: simplify memcg handling in
>  test_clear_page_writeback()
>
> Page writeback doesn't hold a page reference, which allows truncate to
> free a page the second PageWriteback is cleared. This used to require
> special attention in test_clear_page_writeback(), where we had to be
> careful not to rely on the unstable page->memcg binding and look up
> all the necessary information before clearing the writeback flag.
>
> Since commit 073861ed77b6 ("mm: fix VM_BUG_ON(PageTail) and
> BUG_ON(PageWriteback)") test_clear_page_writeback() is called with an
> explicit reference on the page, and this dance is no longer needed.
>
> Use unlock_page_memcg() and dec_lruvec_page_state() directly.
>
> This removes the last user of the lock_page_memcg() return value,
> change it to void. Touch up the comments in there as well. This also
> removes the last extern user of __unlock_page_memcg(), make it
> static. Further, it removes the last user of dec_lruvec_state(),
> delete it, along with a few other unused helpers.
>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Acked-by: Hugh Dickins <hughd@...gle.com>
> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>

The patch looks fine. I don't want to spoil the fun but just wanted to
call out that I might bring back __unlock_page_memcg() for the memcg
accounting of zero copy TCP memory work where we are uncharging the
page in page_remove_rmap().