From: Yu Zhao <yuzhao@google.com>
To: Michal Hocko <mhocko@suse.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
"Alexander Duyck" <alexander.h.duyck@linux.intel.com>,
"Huang Ying" <ying.huang@intel.com>,
"David Hildenbrand" <david@redhat.com>,
"Yang Shi" <yang.shi@linux.alibaba.com>, "Qian Cai" <cai@lca.pw>,
"Mel Gorman" <mgorman@suse.de>,
"Nicholas Piggin" <npiggin@gmail.com>,
"Jérôme Glisse" <jglisse@redhat.com>,
"Hugh Dickins" <hughd@google.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
"Joonsoo Kim" <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH v2 3/3] mm: remove superfluous __ClearPageWaiters()
Date: Thu, 20 Aug 2020 02:12:38 -0600 [thread overview]
Message-ID: <20200820081238.GA2576494@google.com> (raw)
In-Reply-To: <20200820061652.GX5422@dhcp22.suse.cz>
On Thu, Aug 20, 2020 at 08:18:27AM +0200, Michal Hocko wrote:
> On Tue 18-08-20 12:47:04, Yu Zhao wrote:
> > Presumably __ClearPageWaiters() was added to follow the previously
> > removed __ClearPageActive() pattern.
>
> I do not think so. Please have a look at 62906027091f ("mm: add
> PageWaiters indicating tasks are waiting for a page bit") and a
> discussion when the patch has been proposed. Sorry I do not have a link
> handy but I do remember that the handling was quite subtle.
>
> > Only flags that are in PAGE_FLAGS_CHECK_AT_FREE needs to be properly
> > cleared because otherwise we think there may be some kind of leak.
> > PG_waiters is not one of those flags and leaving the clearing to
> > PAGE_FLAGS_CHECK_AT_PREP is more appropriate.
>
> What is the point of this patch in the first place? Page waiters is
> quite subtle and I wouldn't touch it without having a very good reason.
I appreciate your caution. And I just studied the history [1] (I admit
this is something I should have done beforehand), and didn't find any
discussion on __ClearPageWaiters() specifically. So I would ask why it
was added originally. I was hoping Nicholas could help us.
[1] https://lore.kernel.org/lkml/20161225030030.23219-3-npiggin@gmail.com/
Given its triviality, I can't argue how useful this patch is. So I'll
go with how evident it is: we are removing __ClearPageWaiters() from
paths where pages have no references left -- they can't have any
waiters or be on any wait queues.
next prev parent reply other threads:[~2020-08-20 8:13 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-18 18:47 [PATCH v2 1/3] mm: remove activate_page() from unuse_pte() Yu Zhao
2020-08-18 18:47 ` [PATCH v2 2/3] mm: remove superfluous __ClearPageActive() Yu Zhao
2020-08-19 23:04 ` Yang Shi
2020-08-18 18:47 ` [PATCH v2 3/3] mm: remove superfluous __ClearPageWaiters() Yu Zhao
2020-08-19 23:06 ` Yang Shi
2020-08-19 23:39 ` Yu Zhao
2020-08-20 0:16 ` Yang Shi
2020-08-20 6:18 ` Michal Hocko
2020-08-20 8:12 ` Yu Zhao [this message]
2020-08-19 22:16 ` [PATCH v2 1/3] mm: remove activate_page() from unuse_pte() Yang Shi
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