parisc: Optimize per-pagetable spinlocks

On parisc a spinlock is stored in the next page behind the pgd which
protects against parallel accesses to the pgd. That's why one additional
page (PGD_ALLOC_ORDER) is allocated for the pgd.

Matthew Wilcox suggested that we instead should use a pointer in the
struct page table for this spinlock and noted, that the comments for the
PGD_ORDER and PMD_ORDER defines were wrong.

Both suggestions are addressed with this patch. Instead of having an own
spinlock to protect the pgd, we now switch to use the existing
page_table_lock.  Additionally, beside loading the pgd into cr25 in
switch_mm_irqs_off(), the physical address of this lock is loaded into
cr28 (tr4), so that we can avoid implementing a complicated lookup in
assembly for this lock in the TLB fault handlers.

The existing Hybrid L2/L3 page table scheme (where the pmd is adjacent
to the pgd) has been dropped with this patch.

Remove the locking in set_pte() and the huge-page pte functions too.
They trigger a spinlock recursion on 32bit machines and seem unnecessary.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Fixes: b37d1c1898b2 ("parisc: Use per-pagetable spinlock")
Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>

1 files changed, 5 insertions, 5 deletions

diff --git a/arch/parisc/mm/init.c b/arch/parisc/mm/init.c
index 3ec633b11b54..9ca4e4ff6895 100644
--- a/arch/parisc/mm/init.c
+++ b/arch/parisc/mm/init.c
@@ -37,11 +37,6 @@ extern int  data_start;
 extern void parisc_kernel_start(void);	/* Kernel entry point in head.S */
 
 #if CONFIG_PGTABLE_LEVELS == 3
-/* NOTE: This layout exactly conforms to the hybrid L2/L3 page table layout
- * with the first pmd adjacent to the pgd and below it. gcc doesn't actually
- * guarantee that global objects will be laid out in memory in the same order
- * as the order of declaration, so put these in different sections and use
- * the linker script to order them. */
 pmd_t pmd0[PTRS_PER_PMD] __section(".data..vm0.pmd") __attribute__ ((aligned(PAGE_SIZE)));
 #endif
 
@@ -559,6 +554,11 @@ void __init mem_init(void)
 	BUILD_BUG_ON(PGD_ENTRY_SIZE != sizeof(pgd_t));
 	BUILD_BUG_ON(PAGE_SHIFT + BITS_PER_PTE + BITS_PER_PMD + BITS_PER_PGD
 			> BITS_PER_LONG);
+#if CONFIG_PGTABLE_LEVELS == 3
+	BUILD_BUG_ON(PT_INITIAL > PTRS_PER_PMD);
+#else
+	BUILD_BUG_ON(PT_INITIAL > PTRS_PER_PGD);
+#endif
 
 	high_memory = __va((max_pfn << PAGE_SHIFT));
 	set_max_mapnr(max_low_pfn);