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

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Message-ID: <ddc3bb56-4da0-c093-256f-185d4a612b5c@c-s.fr>
Date:   Mon, 10 Sep 2018 14:34:37 +0000
From:   Christophe Leroy <christophe.leroy@....fr>
To:     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: How to handle PTE tables with non contiguous entries ?

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