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Subject: Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages
To:     Michael Ellerman <mpe@ellerman.id.au>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-mm@kvack.org
References: <cover.1592225557.git.christophe.leroy@csgroup.eu>
 <341688399c1b102756046d19ea6ce39db1ae4742.1592225558.git.christophe.leroy@csgroup.eu>
 <20200615132244.GR2531@hirez.programming.kicks-ass.net>
 <87wo45db8d.fsf@mpe.ellerman.id.au>
 <20200617143826.GJ2531@hirez.programming.kicks-ass.net>
 <87pn9xchql.fsf@mpe.ellerman.id.au>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <baa6c8bc-1d2e-d714-8dee-5d97a643f5ee@csgroup.eu>
Date:   Thu, 18 Jun 2020 16:21:16 +0200
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Le 18/06/2020 à 02:58, Michael Ellerman a écrit :
> Peter Zijlstra <peterz@infradead.org> writes:
>> On Thu, Jun 18, 2020 at 12:21:22AM +1000, Michael Ellerman wrote:
>>> Peter Zijlstra <peterz@infradead.org> writes:
>>>> On Mon, Jun 15, 2020 at 12:57:59PM +0000, Christophe Leroy wrote:
>>
>>>>> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
>>>>> +#define __HAVE_ARCH_PTEP_GET
>>>>> +static inline pte_t ptep_get(pte_t *ptep)
>>>>> +{
>>>>> +	pte_t pte = {READ_ONCE(ptep->pte), 0, 0, 0};
>>>>> +
>>>>> +	return pte;
>>>>> +}
>>>>> +#endif
>>>>
>>>> Would it make sense to have a comment with this magic? The casual reader
>>>> might wonder WTH just happened when he stumbles on this :-)
>>>
>>> I tried writing a helpful comment but it's too late for my brain to form
>>> sensible sentences.
>>>
>>> Christophe can you send a follow-up with a comment explaining it? In
>>> particular the zero entries stand out, it's kind of subtle that those
>>> entries are only populated with the right value when we write to the
>>> page table.
>>
>> static inline pte_t ptep_get(pte_t *ptep)
>> {
>> 	unsigned long val = READ_ONCE(ptep->pte);
>> 	/* 16K pages have 4 identical value 4K entries */
>> 	pte_t pte = {val, val, val, val);
>> 	return pte;
>> }
>>
>> Maybe something like that?
> 
> I think val wants to be pte_basic_t, but otherwise yeah I like that much
> better.
> 

I sent a patch for that.

I'll also send one to fix mm/debug_vm_pgtable.c which also uses 
READ_ONCE() to access page table entries.

Christophe