linux-kernel - Re: [PATCH] mm: hugetlb: fix HVO crash on s390

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Message-ID: <4f522b65-1ab8-4725-8da7-3f071e7919c1@redhat.com>
Date: Tue, 28 Oct 2025 13:15:57 -0400
From: Luiz Capitulino <luizcap@...hat.com>
To: Heiko Carstens <hca@...ux.ibm.com>,
 Joao Martins <joao.m.martins@...cle.com>
Cc: osalvador@...e.de, akpm@...ux-foundation.org, david@...hat.com,
 aneesh.kumar@...nel.org, borntraeger@...ux.ibm.com, mike.kravetz@...cle.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH] mm: hugetlb: fix HVO crash on s390

On 2025-10-28 13:02, Heiko Carstens wrote:
> On Tue, Oct 28, 2025 at 04:48:57PM +0000, Joao Martins wrote:
>> On 28/10/2025 16:14, Heiko Carstens wrote:
>>> On Tue, Oct 28, 2025 at 04:05:45PM +0000, Joao Martins wrote:
>>>>> +static inline void vmemmap_flush_tlb_all(void)
>>>>> +{
>>>>> +#ifdef CONFIG_S390
>>>>> +	__tlb_flush_kernel();
>>>>> +#else
>>>>> +	flush_tlb_all();
>>>>> +#endif
>>>>> +}
>>>>> +
>>>>
>>>> Wouldn't a better fix be to implement flush_tlb_all() in
>>>> s390/include/asm/tlbflush.h since that aliases to __tlb_flush_kernel()?
>>>
>>> The question is rather what is flush_tlb_all() supposed to flush? Is
>>> it supposed to flush only tlb entries corresponding to the kernel
>>> address space, or should it flush just everything?
>>>
>> The latter i.e. everything
>>
>> At least as far as I understand
>>
>>> Within this context it looks like only tlb flushing for the kernel
>>> address space is required(?)
>>
>> That's correct. We are changing the vmemmap which is in the kernel address
>> space, so that's the intent.
>>
>> flush_tlb_all() however is the *closest* equivalent to this that's behind an
>> arch generic API i.e. flushing kernel address space on all CPUs TLBs. IIUC, x86
>> when doing flush_tlb_kernel_range with enough pages it switches to flush_tlb_all
>> (these days on modern AMDs it's even one instruction solely in the calling CPU).
> 
> Considering that flush_tlb_all() should be mapped to __tlb_flush_global()
> and not __tlb_flush_kernel() on s390.

You're right.

> However if there is only a need to flush tlb entries for the complete(?)
> kernel address space, then I'd rather propose a new tlb_flush_kernel()
> instead of a big hammer. If I'm not mistaken flush_tlb_kernel_range()
> exists for just avoiding that. And if architectures can avoid a global
> flush of _all_ tlb entries then that should be made possible.

Should we take a v2 doing your suggestion above for now and work on
the tlb_flush_kernel() idea as a follow up improvement? At least we
go from crashing to flushing more than we should...