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From:   Nico Pache <npache@redhat.com>
Date:   Tue, 13 Dec 2022 16:53:30 -0700
Message-ID: <CAA1CXcB653kDtF90oWUgVA3TDUrJHac-WPe1HdsQtRwtN5B3gA@mail.gmail.com>
Subject: Re: [RFC V2] mm: add the zero case to page[1].compound_nr in set_compound_order
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        muchun.song@linux.dev, akpm@linux-foundation.org,
        willy@infradead.org, gerald.schaefer@linux.ibm.com,
        sidhartha.kumar@oracle.com
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Hi Mike,

Thanks for the pointer! Would the branchless conditional be an
improvement over the current approach? I'm not sure how hot this path
is, but it may be worth the optimization.

-- Nico

On Tue, Dec 13, 2022 at 4:48 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 12/13/22 16:45, Nico Pache wrote:
> > Since commit 1378a5ee451a ("mm: store compound_nr as well as
> > compound_order") the page[1].compound_nr must be explicitly set to 0 if
> > calling set_compound_order(page, 0).
> >
> > This can lead to bugs if the caller of set_compound_order(page, 0) forgets
> > to explicitly set compound_nr=0. An example of this is commit ba9c1201beaa
> > ("mm/hugetlb: clear compound_nr before freeing gigantic pages")
>
> There has been some recent work in this area.  The latest patch being,
> https://lore.kernel.org/linux-mm/20221213212053.106058-1-sidhartha.kumar@oracle.com/
>
> --
> Mike Kravetz
>