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

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Message-ID: <20251030145902.16837C8a-hca@linux.ibm.com>
Date: Thu, 30 Oct 2025 15:59:02 +0100
From: Heiko Carstens <hca@...ux.ibm.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Luiz Capitulino <luizcap@...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,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        osalvador@...e.de, david@...hat.com, aneesh.kumar@...nel.org
Subject: Re: [PATCH v2] s390: fix HugeTLB vmemmap optimization crash

On Tue, Oct 28, 2025 at 02:53:34PM -0700, Andrew Morton wrote:
> On Tue, 28 Oct 2025 17:15:33 -0400 Luiz Capitulino <luizcap@...hat.com> wrote:
> > A reproducible crash occurs when enabling HugeTLB vmemmap optimization (HVO)
> > on s390. The crash and the proposed fix were worked on an s390 KVM guest
> > running on an older hypervisor, as I don't have access to an LPAR. However,
> > the same issue should occur on bare-metal.
> > 
> > Reproducer (it may take a few runs to trigger):
> > 
> >  # sysctl vm.hugetlb_optimize_vmemmap=1
> >  # echo 1 > /proc/sys/vm/nr_hugepages
> >  # echo 0 > /proc/sys/vm/nr_hugepages
> > 
> > ...
> > 
> > This commit fixes this by implementing flush_tlb_all() on s390 as an
> > alias to __tlb_flush_global(). This should cause a flush on all TLB
> > entries on all CPUs as expected by the flush_tlb_all() semantics.
> > 
> > ...
> >
> >  arch/s390/include/asm/tlbflush.h | 6 +++++-
> 
> Thanks, I'll add this to mm.git.  If s390 people prefer to merge it
> (or nack it!) then please do so and I'll drop the mm.git copy.

Andrew, could you drop this one please? After looking a bit deeper
into the real problem, this patch would just paper over the real bug
(and it could still happen).

I added you on Cc for the bug fix, but that is supposed to go via the
s390 tree - just in case you are wondering :)