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Subject: Re: [RFC] mm, THP: Map read-only text segments using large THP pages
From:   William Kucharski <william.kucharski@oracle.com>
In-Reply-To: <20180517075740.GA31969@dhcp22.suse.cz>
Date:   Thu, 17 May 2018 08:34:56 -0600
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        linux-fsdevel@vger.kernel.org
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References: <5BB682E1-DD52-4AA9-83E9-DEF091E0C709@oracle.com>
 <20180517075740.GA31969@dhcp22.suse.cz>
To:     Michal Hocko <mhocko@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On May 17, 2018, at 1:57 AM, Michal Hocko <mhocko@kernel.org> wrote:
>=20
> [CCing Kirill and fs-devel]
>=20
> On Mon 14-05-18 07:12:13, William Kucharski wrote:
>> One of the downsides of THP as currently implemented is that it only =
supports
>> large page mappings for anonymous pages.
>=20
> There is a support for shmem merged already. ext4 was next on the plan
> AFAIR but I haven't seen any patches and Kirill was busy with other
> stuff IIRC.

I couldn't find anything that would specifically map text pages with =
large pages,
so perhaps this could be integrated with that or I may have simply =
missed changes
that would ultimately provide that functionality.

>=20
>> I embarked upon this prototype on the theory that it would be =
advantageous to=20
>> be able to map large ranges of read-only text pages using THP as =
well.
>=20
> Can the fs really support THP only for read mappings? What if those
> pages are to be shared in a writable mapping as well? In other words
> can this all work without a full THP support for a particular fs?

The integration with the page cache would indeed require filesystem =
support.

The end result I'd like to see is full R/W support for large THP pages; =
I
thought the RO text mapping proof of concept worthwhile to see what kind =
of
results we might see and what the thoughts of the community were.

Thanks for the feedback.

  -- Bill=