[PATCH v2 3/3] xen/mm: improve freeing of partially scrubbed pages

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

When freeing possibly partially scrubbed pages in populate_physmap() the
whole page is marked as dirty, but that's not fully accurate.  Since the
PGC_need_scrub bit is preserved for the populate_physmap() allocation we
can use those when freeing to detect which pages need scrubbing instead of
marking the whole page as dirty.

This requires exposing free_heap_pages() globally, and switching
populate_physmap() to use it instead of free_domheap_pages().

Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Jan: I'm not sure if that's what you suggested in the review of v1.  I've
added your Suggested-by but I can drop it if that's not what you were
thinking of.
---
 xen/common/memory.c     |  6 +++---
 xen/common/page_alloc.c | 16 +++++++++++++---
 xen/include/xen/mm.h    |  6 ++++++
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/xen/common/memory.c b/xen/common/memory.c
index 1ad4b51c5b02..68eef8291571 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -177,7 +177,7 @@ static void stash_allocation(struct domain *d, struct 
page_info *page,
      * interface is designed to be used for single-threaded domain creation.
      */
     if ( d->pending_scrub || d->is_dying )
-        free_domheap_pages(page, order);
+        free_heap_pages(page, order, false);
     else
     {
         d->pending_scrub_index = scrub_index;
@@ -210,7 +210,7 @@ static struct page_info *get_stashed_allocation(struct 
domain *d,
             *scrub_index = d->pending_scrub_index;
         }
         else
-            free_domheap_pages(d->pending_scrub, d->pending_scrub_order);
+            free_heap_pages(d->pending_scrub, d->pending_scrub_order, false);
 
         /*
          * The caller now owns the page or it has been freed, clear stashed
@@ -391,7 +391,7 @@ static void populate_physmap(struct memop_args *a)
 
                     if ( assign_page(page, a->extent_order, d, memflags) )
                     {
-                        free_domheap_pages(page, a->extent_order);
+                        free_heap_pages(page, a->extent_order, false);
                         goto out;
                     }
                 }
diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index b1edef87124f..8fc9b5a27f1b 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -1529,13 +1529,13 @@ static bool mark_page_free(struct page_info *pg, mfn_t 
mfn)
 static void free_color_heap_page(struct page_info *pg, bool need_scrub);
 
 /* Free 2^@order set of pages. */
-static void free_heap_pages(
-    struct page_info *pg, unsigned int order, bool need_scrub)
+void free_heap_pages(struct page_info *pg, unsigned int order, bool need_scrub)
 {
     unsigned long mask;
     mfn_t mfn = page_to_mfn(pg);
     unsigned int i, node = mfn_to_nid(mfn);
     unsigned int zone = page_to_zone(pg);
+    unsigned int first_dirty = INVALID_DIRTY_IDX, dirty_cnt = 0;
     bool pg_offlined = false;
 
     ASSERT(order <= MAX_ORDER);
@@ -1552,6 +1552,13 @@ static void free_heap_pages(
             pg[i].count_info |= PGC_need_scrub;
             poison_one_page(&pg[i]);
         }
+        else if ( test_bit(_PGC_need_scrub, &pg[i].count_info) )
+        {
+            /* The caller might have returned pages pending scrub. */
+            if ( first_dirty == INVALID_DIRTY_IDX )
+                first_dirty = i;
+            dirty_cnt++;
+        }
 
         if ( pg->count_info & PGC_colored )
         {
@@ -1571,7 +1578,10 @@ static void free_heap_pages(
         pg->u.free.first_dirty = 0;
     }
     else
-        pg->u.free.first_dirty = INVALID_DIRTY_IDX;
+    {
+        node_need_scrub[node] += dirty_cnt;
+        pg->u.free.first_dirty = first_dirty;
+    }
 
     /* Merge chunks as far as possible. */
     while ( order < MAX_ORDER )
diff --git a/xen/include/xen/mm.h b/xen/include/xen/mm.h
index b80bec00c124..0b192caa07bc 100644
--- a/xen/include/xen/mm.h
+++ b/xen/include/xen/mm.h
@@ -153,6 +153,12 @@ unsigned long avail_node_heap_pages(unsigned int nodeid);
 } while ( false )
 #define FREE_DOMHEAP_PAGE(p) FREE_DOMHEAP_PAGES(p, 0)
 
+/*
+ * Most callers should use free_{xen,dom}heap_pages() instead of directly
+ * calling free_heap_pages().
+ */
+void free_heap_pages(struct page_info *pg, unsigned int order, bool 
need_scrub);
+
 void scrub_one_page(const struct page_info *pg, bool cold);
 
 int online_page(mfn_t mfn, uint32_t *status);
-- 
2.51.0