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

LEROY Christophe
2024-06-24T14:24:54Z

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@csgroup.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