linux-kernel - Re: [RFC V2] mm: add the zero case to page[1].compound_nr in set_compound

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

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@...cle.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
>