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From: Michael Ellerman <mpe@ellerman.id.au>
To: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>, Kunwu Chan
 <chentao@kylinos.cn>, npiggin@gmail.com, christophe.leroy@csgroup.eu,
 naveen.n.rao@linux.ibm.com
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] powerpc/mm: Code cleanup for __hash_page_thp
In-Reply-To: <87bk80kjup.fsf@kernel.org>
References: <20240125092624.537564-1-chentao@kylinos.cn>
 <87h6hva4b0.fsf@mail.lhotse>
 <f3b53f0e-58ce-4b2d-ba91-f347da73f9f3@kylinos.cn>
 <87jzmq5tjr.fsf@mail.lhotse> <87bk80kjup.fsf@kernel.org>
Date: Thu, 29 Feb 2024 17:18:32 +1100
Message-ID: <87wmqng5dz.fsf@mail.lhotse>
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Aneesh Kumar K.V <aneesh.kumar@kernel.org> writes:
> Michael Ellerman <mpe@ellerman.id.au> writes:
>> Kunwu Chan <chentao@kylinos.cn> writes:
>>> On 2024/2/26 18:49, Michael Ellerman wrote:
>>>> Kunwu Chan <chentao@kylinos.cn> writes:
>>>>> This part was commented from commit 6d492ecc6489
>>>>> ("powerpc/THP: Add code to handle HPTE faults for hugepages")
>>>>> in about 11 years before.
>>>>>
>>>>> If there are no plans to enable this part code in the future,
>>>>> we can remove this dead code.
>>>> 
>>>> I agree the code can go. But I'd like it to be replaced with a comment
>>>> explaining what the dead code was trying to say.
>>
>>> Thanks, i'll update a new patch with the following comment:
>>>      /*
>>>      * No CPU has hugepages but lacks no execute, so we
>>>      * don't need to worry about cpu no CPU_FTR_COHERENT_ICACHE feature case
>>>      */
>>
>> Maybe wait until we can get some input from Aneesh. I'm not sure the
>> code/comment are really up to date.
>
> How about?
>
> modified   arch/powerpc/mm/book3s64/hash_hugepage.c
> @@ -58,17 +58,13 @@ int __hash_page_thp(unsigned long ea, unsigned long access, unsigned long vsid,
>  		return 0;
>  
>  	rflags = htab_convert_pte_flags(new_pmd, flags);
> +	/*
> +	 * THPs are only supported on platforms that can do mixed page size
> +	 * segments (MPSS) and all such platforms have coherent icache. Hence we
> +	 * don't need to do lazy icache flush (hash_page_do_lazy_icache()) on
> +	 * noexecute fault.
> +	 */

Yeah thanks that looks good.

It could say "see eg. __hash_page_4K()", but that's probably unnecessary
as it mentions hash_page_do_lazy_icache(), and anyone interested is just
going to grep for that anyway.

cheers