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

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Date:   Wed, 10 Feb 2021 19:33:00 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Shakeel Butt <shakeelb@...gle.com>
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 02:59:32PM -0800, Shakeel Butt wrote:
> On Wed, Feb 10, 2021 at 9:44 AM Johannes Weiner <hannes@...xchg.org> wrote:
> > 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().

That shouldn't be an issue. Just add it back if/when you need it and
we have a legitimate in-tree user for it again. It still helps to
remove it now; if someboy later goes through the git log to identify
dependencies, they'll find your patch adding it and can stop looking.

Thanks for the review!