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Date: Wed, 29 May 2024 09:58:35 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Oscar Salvador <osalvador@...e.com>
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: [RFC PATCH v4 13/16] powerpc/e500: Use contiguous PMD instead of
hugepd
Le 29/05/2024 à 10:49, Oscar Salvador a écrit :
> [Vous ne recevez pas souvent de courriers de osalvador@...e.com. D?couvrez pourquoi ceci est important ? https://aka.ms/LearnAboutSenderIdentification ]
>
> On Mon, May 27, 2024 at 03:30:11PM +0200, Christophe Leroy wrote:
>> e500 supports many page sizes among which the following size are
>> implemented in the kernel at the time being: 4M, 16M, 64M, 256M, 1G.
>>
>> On e500, TLB miss for hugepages is exclusively handled by SW even
>> on e6500 which has HW assistance for 4k pages, so there are no
>> constraints like on the 8xx.
>>
>> On e500/32, all are at PGD/PMD level and can be handled as
>> cont-PMD.
>>
>> On e500/64, smaller ones are on PMD while bigger ones are on PUD.
>> Again, they can easily be handled as cont-PMD and cont-PUD instead
>> of hugepd.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>
> ...
>
>> diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h
>> index 90d6a0943b35..f7421d1a1693 100644
>> --- a/arch/powerpc/include/asm/nohash/pgtable.h
>> +++ b/arch/powerpc/include/asm/nohash/pgtable.h
>> @@ -52,11 +52,36 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
>> {
>> pte_basic_t old = pte_val(*p);
>> pte_basic_t new = (old & ~(pte_basic_t)clr) | set;
>> + unsigned long sz;
>> + unsigned long pdsize;
>> + int i;
>>
>> if (new == old)
>> return old;
>>
>> - *p = __pte(new);
>> +#ifdef CONFIG_PPC_E500
>> + if (huge)
>> + sz = 1UL << (((old & _PAGE_HSIZE_MSK) >> _PAGE_HSIZE_SHIFT) + 20);
>> + else
>
> I think this will not compile when CONFIG_PPC_85xx && !CONFIG_PTE_64BIT.
Yes, I got a feedback on this from the robots.
>
> You have declared _PAGE_HSIZE_MSK and _PAGE_HSIZE_SHIFT in
> arch/powerpc/include/asm/nohash/hugetlb-e500.h.
>
> But hugetlb-e500.h is only included if CONFIG_PPC_85xx && CONFIG_PTE_64BIT
> (see arch/powerpc/include/asm/nohash/32/pgtable.h).
>
>
>
>> +#endif
>> + sz = PAGE_SIZE;
>> +
>> + if (!huge || sz < PMD_SIZE)
>> + pdsize = PAGE_SIZE;
>> + else if (sz < PUD_SIZE)
>> + pdsize = PMD_SIZE;
>> + else if (sz < P4D_SIZE)
>> + pdsize = PUD_SIZE;
>> + else if (sz < PGDIR_SIZE)
>> + pdsize = P4D_SIZE;
>> + else
>> + pdsize = PGDIR_SIZE;
>> +
>> + for (i = 0; i < sz / pdsize; i++, p++) {
>> + *p = __pte(new);
>> + if (new)
>> + new += (unsigned long long)(pdsize / PAGE_SIZE) << PTE_RPN_SHIFT;
>
> I guess 'new' can be 0 if pte_update() is called on behave of clearing the pte?
It is exactly that, and without that verification I had pmd_bad()
returning bad pmds after freeing page tables.
>
>> +static inline unsigned long pmd_leaf_size(pmd_t pmd)
>> +{
>> + return 1UL << (((pmd_val(pmd) & _PAGE_HSIZE_MSK) >> _PAGE_HSIZE_SHIFT) + 20);
>
> Can we have the '20' somewhere defined with a comment on top explaining
> what is so it is not a magic number?
> Otherwise people might come look at this and wonder why 20.
Yes I now have :
+#define _PAGE_HSIZE_MSK (_PAGE_U0 | _PAGE_U1 | _PAGE_U2 | _PAGE_U3)
+#define _PAGE_HSIZE_SHIFT 14
+#define _PAGE_HSIZE_SHIFT_OFFSET 20
and have added a helper to avoid doing the calculation at several places:
+static inline unsigned long pte_huge_size(pte_t pte)
+{
+ pte_basic_t val = pte_val(pte);
+
+ return 1UL << (((val & _PAGE_HSIZE_MSK) >> _PAGE_HSIZE_SHIFT) +
_PAGE_HSIZE_SHIFT_OFFSET);
+}
>
>> --- a/arch/powerpc/mm/pgtable.c
>> +++ b/arch/powerpc/mm/pgtable.c
>> @@ -331,6 +331,37 @@ void set_huge_pte_at(struct mm_struct *mm, unsigned long addr, pte_t *ptep,
>> __set_huge_pte_at(pmdp, ptep, pte_val(pte));
>> }
>> }
>> +#elif defined(CONFIG_PPC_E500)
>> +void set_huge_pte_at(struct mm_struct *mm, unsigned long addr, pte_t *ptep,
>> + pte_t pte, unsigned long sz)
>> +{
>> + unsigned long pdsize;
>> + int i;
>> +
>> + pte = set_pte_filter(pte, addr);
>> +
>> + /*
>> + * Make sure hardware valid bit is not set. We don't do
>> + * tlb flush for this update.
>> + */
>> + VM_WARN_ON(pte_hw_valid(*ptep) && !pte_protnone(*ptep));
>> +
>> + if (sz < PMD_SIZE)
>> + pdsize = PAGE_SIZE;
>> + else if (sz < PUD_SIZE)
>> + pdsize = PMD_SIZE;
>> + else if (sz < P4D_SIZE)
>> + pdsize = PUD_SIZE;
>> + else if (sz < PGDIR_SIZE)
>> + pdsize = P4D_SIZE;
>> + else
>> + pdsize = PGDIR_SIZE;
>> +
>> + for (i = 0; i < sz / pdsize; i++, ptep++, addr += pdsize) {
>> + __set_pte_at(mm, addr, ptep, pte, 0);
>> + pte = __pte(pte_val(pte) + ((unsigned long long)pdsize / PAGE_SIZE << PFN_PTE_SHIFT));
>
> You can use pte_advance_pfn() here? Just give have
>
> nr = (unsigned long long)pdsize / PAGE_SIZE << PFN_PTE_SHIFT)
> pte_advance_pfn(pte, nr)
That's what I did before but it didn't work. The problem is that
pte_advance_pfn() takes a long not a long long:
static inline pte_t pte_advance_pfn(pte_t pte, unsigned long nr)
{
return __pte(pte_val(pte) + (nr << PFN_PTE_SHIFT));
}
And when I called it with nr = PMD_SIZE / PAGE_SIZE = 2M / 4k = 512, as
we have PFN_PTE_SHIFT = 24, I got 512 << 24 = 0
>
> Which 'sz's can we have here? You mentioned that e500 support:
>
> 4M, 16M, 64M, 256M, 1G.
>
> which of these ones can be huge?
All are huge.
>
>
> --
> Oscar Salvador
> SUSE Labs
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