xen/p2m: Reuse existing P2M leafs if they are filled with 1:1 PFNs or INVALID.

If P2M leaf is completly packed with INVALID_P2M_ENTRY or with
1:1 PFNs (so IDENTITY_FRAME type PFNs), we can swap the P2M leaf
with either a p2m_missing or p2m_identity respectively. The old
page (which was created via extend_brk or was grafted on from the
mfn_list) can be re-used for setting new PFNs.

This also means we can remove git commit:
5bc6f9888db5739abfa0cae279b4b442e4db8049
xen/p2m: Reserve 8MB of _brk space for P2M leafs when populating back
which tried to fix this.

and make the amount that is required to be reserved much smaller.

CC: stable@vger.kernel.org # for 3.5 only.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index b2e91d40a4cb32a851006d6194d70f3465ec26f3..d4b255463253c8b3bb824a915f5764cc2992e202 100644 (file)
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -196,9 +196,11 @@ RESERVE_BRK(p2m_mid_identity, PAGE_SIZE * 2 * 3);
 
 /* When we populate back during bootup, the amount of pages can vary. The
  * max we have is seen is 395979, but that does not mean it can't be more.
- * But some machines can have 3GB I/O holes even. So lets reserve enough
- * for 4GB of I/O and E820 holes. */
-RESERVE_BRK(p2m_populated, PMD_SIZE * 4);
+ * Some machines can have 3GB I/O holes even. With early_can_reuse_p2m_middle
+ * it can re-use Xen provided mfn_list array, so we only need to allocate at
+ * most three P2M top nodes. */
+RESERVE_BRK(p2m_populated, PAGE_SIZE * 3);
+
 static inline unsigned p2m_top_index(unsigned long pfn)
 {
        BUG_ON(pfn >= MAX_P2M_PFN);
@@ -575,12 +577,99 @@ static bool __init early_alloc_p2m(unsigned long pfn)
        }
        return true;
 }
+
+/*
+ * Skim over the P2M tree looking at pages that are either filled with
+ * INVALID_P2M_ENTRY or with 1:1 PFNs. If found, re-use that page and
+ * replace the P2M leaf with a p2m_missing or p2m_identity.
+ * Stick the old page in the new P2M tree location.
+ */
+bool __init early_can_reuse_p2m_middle(unsigned long set_pfn, unsigned long set_mfn)
+{
+       unsigned topidx;
+       unsigned mididx;
+       unsigned ident_pfns;
+       unsigned inv_pfns;
+       unsigned long *p2m;
+       unsigned long *mid_mfn_p;
+       unsigned idx;
+       unsigned long pfn;
+
+       /* We only look when this entails a P2M middle layer */
+       if (p2m_index(set_pfn))
+               return false;
+
+       for (pfn = 0; pfn <= MAX_DOMAIN_PAGES; pfn += P2M_PER_PAGE) {
+               topidx = p2m_top_index(pfn);
+
+               if (!p2m_top[topidx])
+                       continue;
+
+               if (p2m_top[topidx] == p2m_mid_missing)
+                       continue;
+
+               mididx = p2m_mid_index(pfn);
+               p2m = p2m_top[topidx][mididx];
+               if (!p2m)
+                       continue;
+
+               if ((p2m == p2m_missing) || (p2m == p2m_identity))
+                       continue;
+
+               if ((unsigned long)p2m == INVALID_P2M_ENTRY)
+                       continue;
+
+               ident_pfns = 0;
+               inv_pfns = 0;
+               for (idx = 0; idx < P2M_PER_PAGE; idx++) {
+                       /* IDENTITY_PFNs are 1:1 */
+                       if (p2m[idx] == IDENTITY_FRAME(pfn + idx))
+                               ident_pfns++;
+                       else if (p2m[idx] == INVALID_P2M_ENTRY)
+                               inv_pfns++;
+                       else
+                               break;
+               }
+               if ((ident_pfns == P2M_PER_PAGE) || (inv_pfns == P2M_PER_PAGE))
+                       goto found;
+       }
+       return false;
+found:
+       /* Found one, replace old with p2m_identity or p2m_missing */
+       p2m_top[topidx][mididx] = (ident_pfns ? p2m_identity : p2m_missing);
+       /* And the other for save/restore.. */
+       mid_mfn_p = p2m_top_mfn_p[topidx];
+       /* NOTE: Even if it is a p2m_identity it should still be point to
+        * a page filled with INVALID_P2M_ENTRY entries. */
+       mid_mfn_p[mididx] = virt_to_mfn(p2m_missing);
+
+       /* Reset where we want to stick the old page in. */
+       topidx = p2m_top_index(set_pfn);
+       mididx = p2m_mid_index(set_pfn);
+
+       /* This shouldn't happen */
+       if (WARN_ON(p2m_top[topidx] == p2m_mid_missing))
+               early_alloc_p2m(set_pfn);
+
+       if (WARN_ON(p2m_top[topidx][mididx] != p2m_missing))
+               return false;
+
+       p2m_init(p2m);
+       p2m_top[topidx][mididx] = p2m;
+       mid_mfn_p = p2m_top_mfn_p[topidx];
+       mid_mfn_p[mididx] = virt_to_mfn(p2m);
+
+       return true;
+}
 bool __init early_set_phys_to_machine(unsigned long pfn, unsigned long mfn)
 {
        if (unlikely(!__set_phys_to_machine(pfn, mfn)))  {
                if (!early_alloc_p2m(pfn))
                        return false;
 
+               if (early_can_reuse_p2m_middle(pfn, mfn))
+                       return __set_phys_to_machine(pfn, mfn);
+
                if (!early_alloc_p2m_middle(pfn, false /* boundary crossover OK!*/))
                        return false;