mm: speculative page references

If we can be sure that elevating the page_count on a pagecache page will
pin it, we can speculatively run this operation, and subsequently check to
see if we hit the right page rather than relying on holding a lock or
otherwise pinning a reference to the page.

This can be done if get_page/put_page behaves consistently throughout the
whole tree (ie.  if we "get" the page after it has been used for something
else, we must be able to free it with a put_page).

Actually, there is a period where the count behaves differently: when the
page is free or if it is a constituent page of a compound page.  We need
an atomic_inc_not_zero operation to ensure we don't try to grab the page
in either case.

This patch introduces the core locking protocol to the pagecache (ie.
adds page_cache_get_speculative, and tweaks some update-side code to make
it work).

Thanks to Hugh for pointing out an improvement to the algorithm setting
page_count to zero when we have control of all references, in order to
hold off speculative getters.

[kamezawa.hiroyu@jp.fujitsu.com: fix migration_entry_wait()]
[hugh@veritas.com: fix add_to_page_cache]
[akpm@linux-foundation.org: repair a comment]
Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Jeff Garzik <jeff@garzik.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 54 insertions, 20 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 26672c6cd3ce..0075eac1cd04 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -391,12 +391,10 @@ static pageout_t pageout(struct page *page, struct address_space *mapping,
 }
 
 /*
- * Attempt to detach a locked page from its ->mapping.  If it is dirty or if
- * someone else has a ref on the page, abort and return 0.  If it was
- * successfully detached, return 1.  Assumes the caller has a single ref on
- * this page.
+ * Same as remove_mapping, but if the page is removed from the mapping, it
+ * gets returned with a refcount of 0.
  */
-int remove_mapping(struct address_space *mapping, struct page *page)
+static int __remove_mapping(struct address_space *mapping, struct page *page)
 {
         BUG_ON(!PageLocked(page));
         BUG_ON(mapping != page_mapping(page));
@@ -427,24 +425,24 @@ int remove_mapping(struct address_space *mapping, struct page *page)
          * Note that if SetPageDirty is always performed via set_page_dirty,
          * and thus under tree_lock, then this ordering is not required.
          */
-        if (unlikely(page_count(page) != 2))
+        if (!page_freeze_refs(page, 2))
                 goto cannot_free;
-        smp_rmb();
-        if (unlikely(PageDirty(page)))
+        /* note: atomic_cmpxchg in page_freeze_refs provides the smp_rmb */
+        if (unlikely(PageDirty(page))) {
+                page_unfreeze_refs(page, 2);
                 goto cannot_free;
+        }
 
         if (PageSwapCache(page)) {
                 swp_entry_t swap = { .val = page_private(page) };
                 __delete_from_swap_cache(page);
                 write_unlock_irq(&mapping->tree_lock);
                 swap_free(swap);
-                __put_page(page);       /* The pagecache ref */
-                return 1;
+        } else {
+                __remove_from_page_cache(page);
+                write_unlock_irq(&mapping->tree_lock);
         }
 
-        __remove_from_page_cache(page);
-        write_unlock_irq(&mapping->tree_lock);
-        __put_page(page);
         return 1;
 
 cannot_free:
@@ -453,6 +451,26 @@ cannot_free:
 }
 
 /*
+ * Attempt to detach a locked page from its ->mapping.  If it is dirty or if
+ * someone else has a ref on the page, abort and return 0.  If it was
+ * successfully detached, return 1.  Assumes the caller has a single ref on
+ * this page.
+ */
+int remove_mapping(struct address_space *mapping, struct page *page)
+{
+        if (__remove_mapping(mapping, page)) {
+                /*
+                 * Unfreezing the refcount with 1 rather than 2 effectively
+                 * drops the pagecache ref for us without requiring another
+                 * atomic operation.
+                 */
+                page_unfreeze_refs(page, 1);
+                return 1;
+        }
+        return 0;
+}
+
+/*
  * shrink_page_list() returns the number of reclaimed pages
  */
 static unsigned long shrink_page_list(struct list_head *page_list,
@@ -598,18 +616,34 @@ static unsigned long shrink_page_list(struct list_head *page_list,
                 if (PagePrivate(page)) {
                         if (!try_to_release_page(page, sc->gfp_mask))
                                 goto activate_locked;
-                        if (!mapping && page_count(page) == 1)
-                                goto free_it;
+                        if (!mapping && page_count(page) == 1) {
+                                unlock_page(page);
+                                if (put_page_testzero(page))
+                                        goto free_it;
+                                else {
+                                        /*
+                                         * rare race with speculative reference.
+                                         * the speculative reference will free
+                                         * this page shortly, so we may
+                                         * increment nr_reclaimed here (and
+                                         * leave it off the LRU).
+                                         */
+                                        nr_reclaimed++;
+                                        continue;
+                                }
+                        }
                 }
 
-                if (!mapping || !remove_mapping(mapping, page))
+                if (!mapping || !__remove_mapping(mapping, page))
                         goto keep_locked;
 
-free_it:
                 unlock_page(page);
+free_it:
                 nr_reclaimed++;
-                if (!pagevec_add(&freed_pvec, page))
-                        __pagevec_release_nonlru(&freed_pvec);
+                if (!pagevec_add(&freed_pvec, page)) {
+                        __pagevec_free(&freed_pvec);
+                        pagevec_reinit(&freed_pvec);
+                }
                 continue;
 
 activate_locked:
@@ -623,7 +657,7 @@ keep:
         }
         list_splice(&ret_pages, page_list);
         if (pagevec_count(&freed_pvec))
-                __pagevec_release_nonlru(&freed_pvec);
+                __pagevec_free(&freed_pvec);
         count_vm_events(PGACTIVATE, pgactivate);
         return nr_reclaimed;
 }