linux-kernel - Re: [PATCH v5 16/18] powerpc/64s: Use contiguous PMD/PUD instead of HUGEPD

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Date: Mon, 24 Jun 2024 14:24:51 +0000
From: LEROY Christophe <christophe.leroy2@...soprasteria.com>
To: Oscar Salvador <osalvador@...e.de>
CC: Andrew Morton <akpm@...ux-foundation.org>, Jason Gunthorpe
	<jgg@...dia.com>, Peter Xu <peterx@...hat.com>, Michael Ellerman
	<mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "linuxppc-dev@...ts.ozlabs.org"
	<linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v5 16/18] powerpc/64s: Use contiguous PMD/PUD instead of
 HUGEPD



Le 13/06/2024 à 09:39, Oscar Salvador a écrit :
> On Mon, Jun 10, 2024 at 07:55:01AM +0200, Christophe Leroy wrote:
>> On book3s/64, the only user of hugepd is hash in 4k mode.
>>
>> All other setups (hash-64, radix-4, radix-64) use leaf PMD/PUD.
>>
>> Rework hash-4k to use contiguous PMD and PUD instead.
>>
>> In that setup there are only two huge page sizes: 16M and 16G.
>>
>> 16M sits at PMD level and 16G at PUD level.
>>
>> pte_update doesn't know page size, lets use the same trick as
>> hpte_need_flush() to get page size from segment properties. That's
>> not the most efficient way but let's do that until callers of
>> pte_update() provide page size instead of just a huge flag.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
> ...
>> +static inline unsigned long hash__pte_update(struct mm_struct *mm,
>> +					 unsigned long addr,
>> +					 pte_t *ptep, unsigned long clr,
>> +					 unsigned long set,
>> +					 int huge)
>> +{
>> +	unsigned long old;
>> +
>> +	old = hash__pte_update_one(ptep, clr, set);
>> +
>> +	if (IS_ENABLED(CONFIG_PPC_4K_PAGES) && huge) {
>> +		unsigned int psize = get_slice_psize(mm, addr);
>> +		int nb, i;
>> +
>> +		if (psize == MMU_PAGE_16M)
>> +			nb = SZ_16M / PMD_SIZE;
>> +		else if (psize == MMU_PAGE_16G)
>> +			nb = SZ_16G / PUD_SIZE;
>> +		else
>> +			nb = 1;
> 
> Although that might be a safe default, it might carry consequences down the road?
> It might not, but if we reach that, something went wrong, so I would put a
> WARN_ON_ONCE at least.

Ah right you said it already and I forgot it.

It is now in v6.

> 
>> --- a/arch/powerpc/mm/book3s64/hugetlbpage.c
>> +++ b/arch/powerpc/mm/book3s64/hugetlbpage.c
>> @@ -53,6 +53,16 @@ int __hash_page_huge(unsigned long ea, unsigned long access, unsigned long vsid,
>>   		/* If PTE permissions don't match, take page fault */
>>   		if (unlikely(!check_pte_access(access, old_pte)))
>>   			return 1;
>> +		/*
>> +		 * If hash-4k, hugepages use seeral contiguous PxD entries
>> +		 * so bail out and let mm make the page young or dirty
>> +		 */
>> +		if (IS_ENABLED(CONFIG_PPC_4K_PAGES)) {
>> +			if (!(old_pte & _PAGE_ACCESSED))
>> +				return 1;
>> +			if ((access & _PAGE_WRITE) && !(old_pte & _PAGE_DIRTY))
>> +				return 1;
>> +		}
> 
> You mentioned that we need to bail out otherwise only the first PxD would be
> updated.
> In the comment you say that mm will take care of making the page young
> or dirty.
> Does this mean that the PxDs underneath will not have its bits updated?
>   
> 

No it just means that mm will do it properly by using 
huge_ptep_set_access_flags()

Christophe