linux-kernel - Re: [PATCH] sparc64: fix hugetlb for sun4u

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Message-ID: <04cf759e-9d99-4826-997b-06ede4e7dbd9@oracle.com>
Date: Wed, 16 Jul 2025 09:47:11 -0700
From: Anthony Yznaga <anthony.yznaga@...cle.com>
To: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        sparclinux@...r.kernel.org, davem@...emloft.net, andreas@...sler.com
Cc: linux-kernel@...r.kernel.org, agordeev@...ux.ibm.com, will@...nel.org,
        ryan.roberts@....com, david@...hat.com, osalvador@...e.de
Subject: Re: [PATCH] sparc64: fix hugetlb for sun4u



On 7/16/25 1:41 AM, John Paul Adrian Glaubitz wrote:
> Hi Anthony,
> 
> On Wed, 2025-07-16 at 08:18 +0200, John Paul Adrian Glaubitz wrote:
>> On Tue, 2025-07-15 at 18:24 -0700, Anthony Yznaga wrote:
>>> An attempt to exercise sparc hugetlb code in a sun4u-based guest
>>> running under qemu results in the guest hanging due to being stuck
>>> in a trap loop. This is due to invalid hugetlb TTEs being installed
>>> that do not have the expected _PAGE_PMD_HUGE and page size bits set.
>>> Although the breakage has gone apparently unnoticed for several years,
>>> fix it now so there is the option to exercise sparc hugetlb code under
>>> qemu. This can be useful because sun4v support in qemu does not support
>>> linux guests currently and sun4v-based hardware resources may not be
>>> readily available.
>>
>> It has actually been observed for a long time that newer kernels are
>> unstable on sun4u while there are no stability issues on sun4v.
> 
> Just as a heads-up: I'm currently building a Debian kernel with this patch
> and see if this fixes the stability issues we're seeing on UltraSPARC machines.

If it's the use of hugetlb that is causing problems then this patch 
should help.

> 
>> I'm not sure though whether the Debian kernel enables hugetlb by default.
> 
> We do.
> 
> I'll report back.
> 
> Adrian
>