linux-kernel - Re: How to handle PTE tables with non contiguous entries ?

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Message-Id: <87tvmoh4w9.fsf@linux.ibm.com>
Date:   Mon, 17 Sep 2018 14:33:50 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        aneesh.kumar@...ux.vnet.ibm.com,
        Nicholas Piggin <npiggin@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: How to handle PTE tables with non contiguous entries ?

Christophe Leroy <christophe.leroy@....fr> writes:

> Hi,
>
> I'm having a hard time figuring out the best way to handle the following 
> situation:
>
> On the powerpc8xx, handling 16k size pages requires to have page tables 
> with 4 identical entries.

I assume that hugetlb page size? If so isn't that similar to FSL hugetlb
page table layout?

>
> Initially I was thinking about handling this by simply modifying 
> pte_index() which changing pte_t type in order to have one entry every 
> 16 bytes, then replicate the PTE value at *ptep, *ptep+1,*ptep+2 and 
> *ptep+3 both in set_pte_at() and pte_update().
>
> However, this doesn't work because many many places in the mm core part 
> of the kernel use loops on ptep with single ptep++ increment.
>
> Therefore did it with the following hack:
>
>   /* PTE level */
> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
> +typedef struct { pte_basic_t pte, pte1, pte2, pte3; } pte_t;
> +#else
>   typedef struct { pte_basic_t pte; } pte_t;
> +#endif
>
> @@ -181,7 +192,13 @@ static inline unsigned long pte_update(pte_t *p,
>          : "cc" );
>   #else /* PTE_ATOMIC_UPDATES */
>          unsigned long old = pte_val(*p);
> -       *p = __pte((old & ~clr) | set);
> +       unsigned long new = (old & ~clr) | set;
> +
> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
> +       p->pte = p->pte1 = p->pte2 = p->pte3 = new;
> +#else
> +       *p = __pte(new);
> +#endif
>   #endif /* !PTE_ATOMIC_UPDATES */
>
>   #ifdef CONFIG_44x
>
>
> @@ -161,7 +161,11 @@ static inline void __set_pte_at(struct mm_struct 
> *mm, unsigned long addr,
>          /* Anything else just stores the PTE normally. That covers all 
> 64-bit
>           * cases, and 32-bit non-hash with 32-bit PTEs.
>           */
> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
> +       ptep->pte = ptep->pte1 = ptep->pte2 = ptep->pte3 = pte_val(pte);
> +#else
>          *ptep = pte;
> +#endif
>
>
>
> But I'm not too happy with it as it means pte_t is not a single type 
> anymore so passing it from one function to the other is quite heavy.
>
>
> Would someone have an idea of an elegent way to handle that ?
>
> Thanks
> Christophe

Why would pte_update bother about updating all the 4 entries?. Can you
help me understand the issue?

-aneesh