[Xen-devel] [PATCH] linux: allow use of split page table locks

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

This changes netback to use struct page's mapping field instead of the
index one, and the PageForeign logic the other way around. That way,
CONFIG_SPLIT_PTLOCK_CPUS doesn't have to be suppressed anymore for x86.

After that adjustment, the sparse tree's mm/Kconfig should be identical
to native and could hence be deleted.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

Index: head-2007-02-27/drivers/xen/netback/netback.c
===================================================================
--- head-2007-02-27.orig/drivers/xen/netback/netback.c  2007-02-27 
16:31:12.000000000 +0100
+++ head-2007-02-27/drivers/xen/netback/netback.c       2007-03-01 
15:23:00.000000000 +0100
@@ -40,6 +40,9 @@
 
 /*#define NETBE_DEBUG_INTERRUPT*/
 
+/* extra field used in struct page */
+#define netif_page_index(pg) (*(long *)&(pg)->mapping)
+
 struct netbk_rx_meta {
        skb_frag_t frag;
        int id;
@@ -352,7 +355,7 @@ static u16 netbk_gop_frag(netif_t *netif
                copy_gop->flags = GNTCOPY_dest_gref;
                if (PageForeign(page)) {
                        struct pending_tx_info *src_pend =
-                               &pending_tx_info[page->index];
+                               &pending_tx_info[netif_page_index(page)];
                        copy_gop->source.domid = src_pend->netif->domid;
                        copy_gop->source.u.ref = src_pend->req.gref;
                        copy_gop->flags |= GNTCOPY_source_gref;
@@ -1327,7 +1330,7 @@ static void netif_page_release(struct pa
        /* Ready for next use. */
        init_page_count(page);
 
-       netif_idx_release(page->index);
+       netif_idx_release(netif_page_index(page));
 }
 
 irqreturn_t netif_be_int(int irq, void *dev_id, struct pt_regs *regs)
@@ -1457,7 +1460,7 @@ static int __init netback_init(void)
        for (i = 0; i < MAX_PENDING_REQS; i++) {
                page = mmap_pages[i];
                SetPageForeign(page, netif_page_release);
-               page->index = i;
+               netif_page_index(page) = i;
        }
 
        pending_cons = 0;
Index: head-2007-02-27/include/linux/page-flags.h
===================================================================
--- head-2007-02-27.orig/include/linux/page-flags.h     2007-02-27 
16:26:22.000000000 +0100
+++ head-2007-02-27/include/linux/page-flags.h  2007-03-01 15:15:29.000000000 
+0100
@@ -252,14 +252,14 @@ static inline void SetPageUptodate(struc
 #define PageForeign(page)      test_bit(PG_foreign, &(page)->flags)
 #define SetPageForeign(page, dtor) do {                \
        set_bit(PG_foreign, &(page)->flags);    \
-       (page)->mapping = (void *)dtor;         \
+       (page)->index = (long)(dtor);           \
 } while (0)
 #define ClearPageForeign(page) do {            \
        clear_bit(PG_foreign, &(page)->flags);  \
-       (page)->mapping = NULL;                 \
+       (page)->index = 0;                      \
 } while (0)
 #define PageForeignDestructor(page)            \
-       ( (void (*) (struct page *)) (page)->mapping )(page)
+       ( (void (*) (struct page *)) (page)->index )(page)
 
 struct page;   /* forward declaration */
 
Index: head-2007-02-27/mm/Kconfig
===================================================================
--- head-2007-02-27.orig/mm/Kconfig     2007-02-27 16:23:14.000000000 +0100
+++ head-2007-02-27/mm/Kconfig  2007-03-01 15:16:15.000000000 +0100
@@ -127,14 +127,11 @@ config MEMORY_HOTPLUG_SPARSE
 # Default to 4 for wider testing, though 8 might be more appropriate.
 # ARM's adjust_pte (unused if VIPT) depends on mm-wide page_table_lock.
 # PA-RISC 7xxx's spinlock_t would enlarge struct page from 32 to 44 bytes.
-# XEN on x86 architecture uses the mapping field on pagetable pages to store a
-# pointer to the destructor. This conflicts with pte_lock_deinit().
 #
 config SPLIT_PTLOCK_CPUS
        int
        default "4096" if ARM && !CPU_CACHE_VIPT
        default "4096" if PARISC && !PA20
-       default "4096" if X86_XEN || X86_64_XEN
        default "4"
 
 #



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