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From:   Yang Shi <shy828301@gmail.com>
Date:   Wed, 19 Aug 2020 17:16:49 -0700
Message-ID: <CAHbLzkp4eOf=jywp+wuqqJoJwqMt7338cfxUsfz1KSt=zEZUiw@mail.gmail.com>
Subject: Re: [PATCH v2 3/3] mm: remove superfluous __ClearPageWaiters()
To:     Yu Zhao <yuzhao@google.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>,
        Michal Hocko <mhocko@suse.com>,
        Yang Shi <yang.shi@linux.alibaba.com>, Qian Cai <cai@lca.pw>,
        Mel Gorman <mgorman@suse.de>,
        Nicholas Piggin <npiggin@gmail.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Hugh Dickins <hughd@google.com>, Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
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On Wed, Aug 19, 2020 at 4:39 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Wed, Aug 19, 2020 at 04:06:32PM -0700, Yang Shi wrote:
> > On Tue, Aug 18, 2020 at 11:47 AM Yu Zhao <yuzhao@google.com> wrote:
> > >
> > > Presumably __ClearPageWaiters() was added to follow the previously
> > > removed __ClearPageActive() pattern.
> > >
> > > 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.
> >
> > Actually TBH I'm not very keen to this change, it seems the clearing
> > is just moved around and the allocation side pays for that instead of
> > free side.
>
> I'll assume you are referring to the overhead from clearing
> PG_waiters. First of all, there is no overhead -- we should have a
> serious talk with the hardware team who makes word-size bitwise AND
> more than one instruction. And the clearing is done in
> free_pages_prepare(), which has nothing to do with allocations.

Oh, yes, you are right. Now I'm wondering why we have the waiter bit
cleared at the first place.