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

Le 18/09/2018 à 13:47, Aneesh Kumar K.V a écrit :

Christophe LEROY [off-list ref] writes:

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Le 17/09/2018 à 11:03, Aneesh Kumar K.V a écrit :

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Christophe Leroy [off-list ref] writes:

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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?

No, it is not for 16k hugepage size with a standard page size of 4k.

Here I'm trying to handle the case of CONFIG_PPC_16K_PAGES.
As of today, it is implemented by using the standard Linux page layout,
ie one PTE entry for each 16k page. This forbids the use the 8xx HW
assistance.

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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?

Because the 8xx HW assistance expects 4 identical entries for each 16k
page, so everytime a PTE is updated the 4 entries have to be updated.

What you suggested in the original mail is what matches that best isn't it?
That is a linux pte update involves updating 4 slot. Hence a linux pte
consist of 4 unsigned long?

Yes indeed.
It seems hopeless to avoid carrying the 4 longs from one function to the 
other allthough that's four times the same thing.

Christophe