linux-kernel - Re: [PATCH v2] s390: fix HugeTLB vmemmap optimization crash

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Message-ID: <bdd4ea3a-f326-4eb8-866a-52b1ea2bdc5a@redhat.com>
Date: Thu, 30 Oct 2025 10:54:47 -0400
From: Luiz Capitulino <luizcap@...hat.com>
To: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>
Cc: David Hildenbrand <david@...hat.com>, borntraeger@...ux.ibm.com,
 joao.m.martins@...cle.com, mike.kravetz@...cle.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-s390@...r.kernel.org, gor@...ux.ibm.com, agordeev@...ux.ibm.com,
 osalvador@...e.de, akpm@...ux-foundation.org, aneesh.kumar@...nel.org
Subject: Re: [PATCH v2] s390: fix HugeTLB vmemmap optimization crash

On 2025-10-30 10:38, Gerald Schaefer wrote:
> On Wed, 29 Oct 2025 13:49:53 +0100
> Heiko Carstens <hca@...ux.ibm.com> wrote:
> 
>> On Wed, Oct 29, 2025 at 01:15:44PM +0100, David Hildenbrand wrote:
>>> BTW, I'm staring at s390x's flush_tlb() function and wonder why that one is
>>> defined. I'm sure there is a good reason ;)
>>
>> Yes, I stumbled across that yesterday evening as well. I think its only
>> purpose is that it wants to be deleted :). I just didn't do it yet since I
>> don't want to see a merge conflict with this patch.
>>
>> I also need to check if the only usage of flush_tlb_page(), which is also a
>> no-op for s390, in mm/memory.c is not indicating a problem too.
>>
>>>> Changing active entries without the detour over an invalid entry or using
>>>> proper instructions like crdte or cspg is not allowed on s390. This was solved
>>>> for other parts that change active entries of the kernel mapping in an
>>>> architecture compliant way for s390 (see arch/s390/mm/pageattr.c).
>>>
>>> Good point. I recall ARM64 has similar break-before-make requirements
>>> because they cannot tolerate two different TLB entries (small vs. large) for
>>> the same virtual address.
>>>
>>> And if I rememebr correctly, that's the reason why arm64 does not enable
>>> ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP just yet.
>>
>> Ok, let's wait for Gerald. Maybe there is a non-obvious reason why this works
>> anyway.
> 
> No, using pmd_populate_kernel() on an active/valid PMD in vmemmap_split_pmd()
> should violate the architecture, as you described. So this would not work
> with current code, and also should not have worked when I did the change,
> or only by chance.
> 
> Therefore, we should disable ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP again, for
> now. Doing it right would most likely require common code changes and
> CRDTE / CSPG usage on s390. Not sure if this feature is really worth the
> hassle, reading all the drawbacks that I mentioned in my commit 00a34d5a99c0
> ("s390: select ARCH_WANT_HUGETLB_PAGE_OPTIMIZE_VMEMMAP").

OK, let's do the right thing. Do you plan to post a patch? I can do it
if you would like.