[Xen-devel] [PATCH] linux/x86: fix and improve xen_limit_pages_to_max_mfn()

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

- don't do multicall when nr_mcl is zero (and specifically don't access
  cr_mcl[nr_mcl - 1] in that case)
- fix CONFIG_XEN_COMPAT <= 0x030002 handling
- don't exchange pages already meeting the restriction (likely avoiding
  exchanging anything at all)
- avoid calling kmap functions without CONFIG_XEN_SCRUB_PAGES
- eliminate a few local variables

As usual, written and tested on 2.6.24 and made apply to the 2.6.18
tree without further testing.

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

Index: head-2008-01-28/arch/i386/mm/hypervisor.c
===================================================================
--- head-2008-01-28.orig/arch/i386/mm/hypervisor.c      2008-02-08 
11:30:34.000000000 +0100
+++ head-2008-01-28/arch/i386/mm/hypervisor.c   2008-02-08 14:35:09.000000000 
+0100
@@ -432,19 +432,17 @@ int xen_limit_pages_to_max_mfn(
 {
        unsigned long flags, frame;
        unsigned long *in_frames = discontig_frames, *out_frames = 
limited_frames;
-       void *v;
        struct page *page;
-       unsigned int i, nr_mcl;
+       unsigned int i, n, nr_mcl;
        int rc, success;
+       DECLARE_BITMAP(limit_map, 1 << MAX_CONTIG_ORDER);
 
        struct xen_memory_exchange exchange = {
                .in = {
-                       .nr_extents   = 1UL << order,
                        .extent_order = 0,
                        .domid        = DOMID_SELF
                },
                .out = {
-                       .nr_extents   = 1UL << order,
                        .extent_order = 0,
                        .address_bits = address_bits,
                        .domid        = DOMID_SELF
@@ -457,80 +455,98 @@ int xen_limit_pages_to_max_mfn(
        if (unlikely(order > MAX_CONTIG_ORDER))
                return -ENOMEM;
 
+       bitmap_zero(limit_map, 1U << order);
        set_xen_guest_handle(exchange.in.extent_start, in_frames);
        set_xen_guest_handle(exchange.out.extent_start, out_frames);
 
        /* 0. Scrub the pages. */
-       for ( i = 0 ; i < 1UL<<order ; i++ ) {
+       for (i = 0, n = 0; i < 1U<<order ; i++) {
                page = &pages[i];
+               if (!(pfn_to_mfn(page_to_pfn(page)) >> (address_bits - 
PAGE_SHIFT)))
+                       continue;
+               __set_bit(i, limit_map);
 
-               if (!PageHighMem(page)) {
-                       v = page_address(page);
-                       scrub_pages(v, 1);
-               } else {
-                       v = kmap(page);
-                       scrub_pages(v, 1);
+               if (!PageHighMem(page))
+                       scrub_pages(page_address(page), 1);
+#ifdef CONFIG_XEN_SCRUB_PAGES
+               else {
+                       scrub_pages(kmap(page), 1);
                        kunmap(page);
+                       ++n;
                }
+#endif
        }
+       if (bitmap_empty(limit_map, 1U << order))
+               return 0;
 
-       kmap_flush_unused();
+       if (n)
+               kmap_flush_unused();
 
        balloon_lock(flags);
 
        /* 1. Zap current PTEs (if any), remembering MFNs. */
-       for (i = 0, nr_mcl = 0; i < (1U<<order); i++) {
+       for (i = 0, n = 0, nr_mcl = 0; i < (1U<<order); i++) {
+               if(!test_bit(i, limit_map))
+                       continue;
                page = &pages[i];
 
-               out_frames[i] = page_to_pfn(page);
-               in_frames[i] = pfn_to_mfn(out_frames[i]);
+               out_frames[n] = page_to_pfn(page);
+               in_frames[n] = pfn_to_mfn(out_frames[n]);
 
                if (!PageHighMem(page))
                        MULTI_update_va_mapping(cr_mcl + nr_mcl++,
                                                (unsigned 
long)page_address(page),
                                                __pte_ma(0), 0);
 
-               set_phys_to_machine(out_frames[i], INVALID_P2M_ENTRY);
+               set_phys_to_machine(out_frames[n], INVALID_P2M_ENTRY);
+               ++n;
        }
-       if (HYPERVISOR_multicall_check(cr_mcl, nr_mcl, NULL))
+       if (nr_mcl && HYPERVISOR_multicall_check(cr_mcl, nr_mcl, NULL))
                BUG();
 
        /* 2. Get new memory below the required limit. */
+       exchange.in.nr_extents = n;
+       exchange.out.nr_extents = n;
        rc = HYPERVISOR_memory_op(XENMEM_exchange, &exchange);
-       success = (exchange.nr_exchanged == (1UL << order));
+       success = (exchange.nr_exchanged == n);
        BUG_ON(!success && ((exchange.nr_exchanged != 0) || (rc == 0)));
        BUG_ON(success && (rc != 0));
 #if CONFIG_XEN_COMPAT <= 0x030002
        if (unlikely(rc == -ENOSYS)) {
                /* Compatibility when XENMEM_exchange is unsupported. */
                if (HYPERVISOR_memory_op(XENMEM_decrease_reservation,
-                                        &exchange.in) != (1UL << order))
+                                        &exchange.in) != n)
+                       BUG();
+               if (HYPERVISOR_memory_op(XENMEM_populate_physmap,
+                                        &exchange.out) != n)
                        BUG();
-               success = (HYPERVISOR_memory_op(XENMEM_populate_physmap,
-                                               &exchange.out) != (1UL 
<<order));
+               success = 1;
        }
 #endif
 
        /* 3. Map the new pages in place of old pages. */
-       for (i = 0, nr_mcl = 0; i < (1U<<order); i++) {
-               unsigned long pfn;
+       for (i = 0, n = 0, nr_mcl = 0; i < (1U<<order); i++) {
+               if(!test_bit(i, limit_map))
+                       continue;
                page = &pages[i];
-               pfn = page_to_pfn(page);
 
-               frame = success ? out_frames[i] : in_frames[i];
+               frame = success ? out_frames[n] : in_frames[n];
 
                if (!PageHighMem(page))
                        MULTI_update_va_mapping(cr_mcl + nr_mcl++,
                                                (unsigned 
long)page_address(page),
                                                pfn_pte_ma(frame, PAGE_KERNEL), 
0);
 
-               set_phys_to_machine(pfn, frame);
+               set_phys_to_machine(page_to_pfn(page), frame);
+               ++n;
+       }
+       if (nr_mcl) {
+               cr_mcl[nr_mcl - 1].args[MULTI_UVMFLAGS_INDEX] = order
+                                                               ? 
UVMF_TLB_FLUSH|UVMF_ALL
+                                                               : 
UVMF_INVLPG|UVMF_ALL;
+               if (HYPERVISOR_multicall_check(cr_mcl, nr_mcl, NULL))
+                       BUG();
        }
-       cr_mcl[nr_mcl - 1].args[MULTI_UVMFLAGS_INDEX] = order
-                                                       ? 
UVMF_TLB_FLUSH|UVMF_ALL
-                                                       : UVMF_INVLPG|UVMF_ALL;
-       if (HYPERVISOR_multicall_check(cr_mcl, nr_mcl, NULL))
-               BUG();
 
        balloon_unlock(flags);
 



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