From: Qian Cai <cai@lca.pw>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Linux Memory Management List <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
Mike Rapoport <rppt@linux.ibm.com>,
Vineet Gupta <vgupta@synopsys.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Heiko Carstens <heiko.carstens@de.ibm.com>,
Vasily Gorbik <gor@linux.ibm.com>,
Christian Borntraeger <borntraeger@de.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>,
"Kirill A . Shutemov" <kirill@shutemov.name>,
Paul Walmsley <paul.walmsley@sifive.com>,
Palmer Dabbelt <palmer@dabbelt.com>,
linux-snps-arc@lists.infradead.org,
linux-arm-kernel@lists.infradead.org,
linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
linux-riscv@lists.infradead.org, x86@kernel.org,
linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
Christophe Leroy <christophe.leroy@c-s.fr>
Subject: Re: [PATCH V15] mm/debug: Add tests validating architecture page table helpers
Date: Fri, 6 Mar 2020 20:10:52 -0500 [thread overview]
Message-ID: <C65A91AB-3F70-4120-893D-2751601C0F25@lca.pw> (raw)
In-Reply-To: <a45834bc-e6f2-ac21-de9e-1aff67d12797@arm.com>
> On Mar 6, 2020, at 7:56 PM, Anshuman Khandual <anshuman.khandual@arm.com> wrote:
>
>
>
> On 03/07/2020 06:04 AM, Qian Cai wrote:
>>
>>
>>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@arm.com> wrote:
>>>
>>> Hmm, set_pte_at() function is not preferred here for these tests. The idea
>>> is to avoid or atleast minimize TLB/cache flushes triggered from these sort
>>> of 'static' tests. set_pte_at() is platform provided and could/might trigger
>>> these flushes or some other platform specific synchronization stuff. Just
>>
>> Why is that important for this debugging option?
>
> Primarily reason is to avoid TLB/cache flush instructions on the system
> during these tests that only involve transforming different page table
> level entries through helpers. Unless really necessary, why should it
> emit any TLB/cache flush instructions ?
>
>>
>>> wondering is there specific reason with respect to the soft lock up problem
>>> making it necessary to use set_pte_at() rather than a simple WRITE_ONCE() ?
>>
>> Looks at the s390 version of set_pte_at(), it has this comment,
>> vmaddr);
>>
>> /*
>> * Certain architectures need to do special things when PTEs
>> * within a page table are directly modified. Thus, the following
>> * hook is made available.
>> */
>>
>> I can only guess that powerpc could be the same here.
>
> This comment is present in multiple platforms while defining set_pte_at().
> Is not 'barrier()' here alone good enough ? Else what exactly set_pte_at()
No, barrier() is not enough.
> does as compared to WRITE_ONCE() that avoids the soft lock up, just trying
> to understand.
I surely can spend hours to figure which exact things in set_pte_at() is necessary for
pte_clear() not to stuck, and then propose a solution and possible need to retest on
multiple arches. I am not sure if that is a good use of my time just to saving
a few TLB/cache flush on a debug kernel?
next prev parent reply other threads:[~2020-03-07 1:10 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-05 23:57 [PATCH V15] mm/debug: Add tests validating architecture page table helpers Anshuman Khandual
2020-03-06 20:44 ` Qian Cai
2020-03-07 0:03 ` Anshuman Khandual
2020-03-07 0:34 ` Qian Cai
2020-03-07 0:56 ` Anshuman Khandual
2020-03-07 1:10 ` Qian Cai [this message]
2020-03-07 7:05 ` Christophe Leroy
2020-03-10 2:31 ` Anshuman Khandual
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