How to handle PTE tables with non contiguous entries ?

From: Christophe Leroy
Date: Mon Sep 10 2018 - 10:46:50 EST


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.

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