rmap: add exclusively owned pages to the newest anon_vma

The recent anon_vma fixes cause many anonymous pages to end up
in the parent process anon_vma, even when the page is exclusively
owned by the current process.

Adding exclusively owned anonymous pages to the top anon_vma
reduces rmap scanning overhead, especially in workloads with
forking servers.

This patch adds a parameter to __page_set_anon_rmap that can
be used to indicate whether or not the added page is exclusively
owned by the current process.

Pages added through page_add_new_anon_rmap are exclusively
owned by the current process, and can be added to the top
anon_vma.

Pages added through page_add_anon_rmap can be either shared
or exclusively owned, so we do the conservative thing and
add it to the oldest anon_vma.

A next step would be to add the exclusive parameter to
page_add_anon_rmap, to be used from functions where we do
know for sure whether a page is exclusively owned.

Signed-off-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Lightly-tested-by: Borislav Petkov <bp@alien8.de>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
[ Edited to look nicer  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 11 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 4bad3267537a..526704e8215d 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -730,23 +730,28 @@ void page_move_anon_rmap(struct page *page,
  * @page:       the page to add the mapping to
  * @vma:        the vm area in which the mapping is added
  * @address:    the user virtual address mapped
+ * @exclusive:  the page is exclusively owned by the current process
  */
 static void __page_set_anon_rmap(struct page *page,
-        struct vm_area_struct *vma, unsigned long address)
+        struct vm_area_struct *vma, unsigned long address, int exclusive)
 {
-        struct anon_vma_chain *avc;
-        struct anon_vma *anon_vma;
+        struct anon_vma *anon_vma = vma->anon_vma;
 
-        BUG_ON(!vma->anon_vma);
+        BUG_ON(!anon_vma);
 
         /*
-         * We must use the _oldest_ possible anon_vma for the page mapping!
+         * If the page isn't exclusively mapped into this vma,
+         * we must use the _oldest_ possible anon_vma for the
+         * page mapping!
          *
-         * So take the last AVC chain entry in the vma, which is the deepest
-         * ancestor, and use the anon_vma from that.
+         * So take the last AVC chain entry in the vma, which is
+         * the deepest ancestor, and use the anon_vma from that.
          */
-        avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
-        anon_vma = avc->anon_vma;
+        if (!exclusive) {
+                struct anon_vma_chain *avc;
+                avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
+                anon_vma = avc->anon_vma;
+        }
 
         anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
         page->mapping = (struct address_space *) anon_vma;
@@ -802,7 +807,7 @@ void page_add_anon_rmap(struct page *page,
         VM_BUG_ON(!PageLocked(page));
         VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
         if (first)
-                __page_set_anon_rmap(page, vma, address);
+                __page_set_anon_rmap(page, vma, address, 0);
         else
                 __page_check_anon_rmap(page, vma, address);
 }
@@ -824,7 +829,7 @@ void page_add_new_anon_rmap(struct page *page,
         SetPageSwapBacked(page);
         atomic_set(&page->_mapcount, 0); /* increment count (starts at -1) */
         __inc_zone_page_state(page, NR_ANON_PAGES);
-        __page_set_anon_rmap(page, vma, address);
+        __page_set_anon_rmap(page, vma, address, 1);
         if (page_evictable(page, vma))
                 lru_cache_add_lru(page, LRU_ACTIVE_ANON);
         else