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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH V2] hugetlb: Add nohugepages parameter to prevent hugepages creation
Date:   Tue, 15 Oct 2019 04:38:33 -0400
Message-Id: <3340809E-6529-4A14-9D42-B14D7E54A8F3@lca.pw>
References: <20191015045158.5297-1-gpiccoli@canonical.com>
Cc:     linux-mm@kvack.org, mike.kravetz@oracle.com,
        linux-kernel@vger.kernel.org, jay.vosburgh@canonical.com,
        kernel@gpiccoli.net
In-Reply-To: <20191015045158.5297-1-gpiccoli@canonical.com>
To:     "Guilherme G. Piccoli" <gpiccoli@canonical.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Oct 15, 2019, at 12:52 AM, Guilherme G. Piccoli <gpiccoli@canonical.com=
> wrote:
>=20
> Kdump kernels won't benefit from hugepages - in fact it's quite opposite,
> it may be the case hugepages on kdump kernel can lead to OOM if kernel
> gets unable to allocate demanded pages due to the fact the preallocated
> hugepages are consuming a lot of memory.
>=20
> This patch proposes a new kernel parameter to prevent the creation of
> HugeTLB hugepages - we currently don't have a way to do that. We can
> even have kdump scripts removing the kernel command-line options to
> set hugepages, but it's not straightforward to prevent sysctl/sysfs
> configuration, given it happens in later boot or anytime when the
> system is running.
>=20
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
> ---
> .../admin-guide/kernel-parameters.txt         |  4 +++
> fs/hugetlbfs/inode.c                          |  5 ++--
> include/linux/hugetlb.h                       |  7 ++++++
> mm/hugetlb.c                                  | 25 +++++++++++++------
> 4 files changed, 32 insertions(+), 9 deletions(-)
>=20
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentati=
on/admin-guide/kernel-parameters.txt
> index a84a83f8881e..061bec851114 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2982,6 +2982,10 @@
>=20
>    nohugeiomap    [KNL,x86,PPC] Disable kernel huge I/O mappings.
>=20
> +    nohugepages    [KNL] Disable HugeTLB hugepages completely, preventing=

> +            its setting either by kernel parameter or sysfs;
> +            useful specially in kdump kernel.
> +
>    nosmt        [KNL,S390] Disable symmetric multithreading (SMT).
>            Equivalent to smt=3D1.

No, it does make much sense to even mention kdump here at all as a justifica=
tion. If somebody blindly set sysfs in the kdump kernel, it will be garbage i=
n and garbage out. It is trivial enough to disable it, as it is what have be=
en done in enterprise distros implementation of kdump where it has no such p=
roblem for a decade.=