linux-kernel - Re: [PATCH 3/3] powerpc/8xx: Provide ptep

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Date:   Thu, 18 Jun 2020 16:21:16 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages



Le 18/06/2020 à 02:58, Michael Ellerman a écrit :
> Peter Zijlstra <peterz@...radead.org> writes:
>> On Thu, Jun 18, 2020 at 12:21:22AM +1000, Michael Ellerman wrote:
>>> Peter Zijlstra <peterz@...radead.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