[POWERPC] Small fixes & cleanups in segment page size demotion

The code for demoting segments to 4K had some issues, like for example,
when using _PAGE_4K_PFN flag, the first CPU to hit it would do the
demotion, but other CPUs hitting the same page wouldn't properly flush
their SLBs if mmu_ci_restriction isn't set.  There are also potential
issues with hash_preload not handling _PAGE_4K_PFN.  All of these are
non issues on current hardware but might bite us in the future.

This patch thus fixes it by:

 - Taking the test comparing the mm and current CPU context page
sizes to decide to flush SLBs out of the mmu_ci_restrictions test
since that can also be triggered by _PAGE_4K_PFN pages

 - Due to the above being done all the time, demote_segment_4k
doesn't need update the context and flush the SLB

 - demote_segment_4k can be static and doesn't need an EXPORT_SYMBOL

 - Making hash_preload ignore anything that has either _PAGE_4K_PFN
or _PAGE_NO_CACHE set, thus avoiding duplication of the complicated
logic in hash_page() (and possibly making hash_preload a little bit
faster for the normal case).
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/mm/hash_utils_64.c

@@ -596,22 +596,18 @@ unsigned int hash_page_do_lazy_icache(unsigned int pp, pte_t pte, int trap)
  * Demote a segment to using 4k pages.
  * For now this makes the whole process use 4k pages.
  */
-void demote_segment_4k(struct mm_struct *mm, unsigned long addr)
-{
 #ifdef CONFIG_PPC_64K_PAGES
+static void demote_segment_4k(struct mm_struct *mm, unsigned long addr)
+{
 	if (mm->context.user_psize == MMU_PAGE_4K)
 		return;
 	mm->context.user_psize = MMU_PAGE_4K;
 	mm->context.sllp = SLB_VSID_USER | mmu_psize_defs[MMU_PAGE_4K].sllp;
-	get_paca()->context = mm->context;
-	slb_flush_and_rebolt();
 #ifdef CONFIG_SPE_BASE
 	spu_flush_all_slbs(mm);
 #endif
-#endif
 }
-
-EXPORT_SYMBOL_GPL(demote_segment_4k);
+#endif /* CONFIG_PPC_64K_PAGES */
 
 /* Result code is:
  *  0 - handled
@@ -711,9 +707,10 @@ int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
 		psize = MMU_PAGE_4K;
 	}
 
-	if (mmu_ci_restrictions) {
-		/* If this PTE is non-cacheable, switch to 4k */
-		if (psize == MMU_PAGE_64K &&
+	/* If this PTE is non-cacheable and we have restrictions on
+	 * using non cacheable large pages, then we switch to 4k
+	 */
+	if (mmu_ci_restrictions && psize == MMU_PAGE_64K &&
 	    (pte_val(*ptep) & _PAGE_NO_CACHE)) {
 		if (user_region) {
 			demote_segment_4k(mm, ea);
@@ -728,14 +725,15 @@ int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
 			       "to 4kB pages because of "
 			       "non-cacheable mapping\n");
 			psize = mmu_vmalloc_psize = MMU_PAGE_4K;
-			}
 #ifdef CONFIG_SPE_BASE
 			spu_flush_all_slbs(mm);
 #endif
 		}
+	}
 	if (user_region) {
 		if (psize != get_paca()->context.user_psize) {
-				get_paca()->context = mm->context;
+			get_paca()->context.user_psize =
+				mm->context.user_psize;
 			slb_flush_and_rebolt();
 		}
 	} else if (get_paca()->vmalloc_sllp !=
@@ -744,7 +742,7 @@ int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
 			mmu_psize_defs[mmu_vmalloc_psize].sllp;
 		slb_flush_and_rebolt();
 	}
-	}
+
 	if (psize == MMU_PAGE_64K)
 		rc = __hash_page_64K(ea, access, vsid, ptep, trap, local);
 	else
@@ -780,13 +778,26 @@ void hash_preload(struct mm_struct *mm, unsigned long ea,
 	DBG_LOW("hash_preload(mm=%p, mm->pgdir=%p, ea=%016lx, access=%lx,"
 		" trap=%lx\n", mm, mm->pgd, ea, access, trap);
 
-	/* Get PTE, VSID, access mask */
+	/* Get Linux PTE if available */
 	pgdir = mm->pgd;
 	if (pgdir == NULL)
 		return;
 	ptep = find_linux_pte(pgdir, ea);
 	if (!ptep)
 		return;
+
+#ifdef CONFIG_PPC_64K_PAGES
+	/* If either _PAGE_4K_PFN or _PAGE_NO_CACHE is set (and we are on
+	 * a 64K kernel), then we don't preload, hash_page() will take
+	 * care of it once we actually try to access the page.
+	 * That way we don't have to duplicate all of the logic for segment
+	 * page size demotion here
+	 */
+	if (pte_val(*ptep) & (_PAGE_4K_PFN | _PAGE_NO_CACHE))
+		return;
+#endif /* CONFIG_PPC_64K_PAGES */
+
+	/* Get VSID */
 	vsid = get_vsid(mm->context.id, ea);
 
 	/* Hash it in */
@@ -797,12 +808,6 @@ void hash_preload(struct mm_struct *mm, unsigned long ea,
 #ifndef CONFIG_PPC_64K_PAGES
 	__hash_page_4K(ea, access, vsid, ptep, trap, local);
 #else
-	if (mmu_ci_restrictions) {
-		/* If this PTE is non-cacheable, switch to 4k */
-		if (mm->context.user_psize == MMU_PAGE_64K &&
-		    (pte_val(*ptep) & _PAGE_NO_CACHE))
-			demote_segment_4k(mm, ea);
-	}
 	if (mm->context.user_psize == MMU_PAGE_64K)
 		__hash_page_64K(ea, access, vsid, ptep, trap, local);
 	else