[patch 3/7] mm: speculative page references

npiggin
2008-06-05T09:43:03Z

[-- Attachment #1: mm-speculative-get_page-hugh.patch --]
[-- Type: text/plain, Size: 13116 bytes --]

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.

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/include/linux/pagemap.h
===================================================================
--- linux-2.6.orig/include/linux/pagemap.h
+++ linux-2.6/include/linux/pagemap.h
@@ -12,6 +12,7 @@
#include <asm/uaccess.h>
#include <linux/gfp.h>
#include <linux/bitops.h>
+#include <linux/hardirq.h> /* for in_interrupt() */

/*
* Bits in mapping->flags. The lower __GFP_BITS_SHIFT bits are the page
@@ -62,6 +63,98 @@ static inline void mapping_set_gfp_mask(
#define page_cache_release(page) put_page(page)
void release_pages(struct page **pages, int nr, int cold);

+/*
+ * speculatively take a reference to a page.
+ * If the page is free (_count == 0), then _count is untouched, and 0
+ * is returned. Otherwise, _count is incremented by 1 and 1 is returned.
+ *
+ * This function must be called inside the same rcu_read_lock() section as has
+ * been used to lookup the page in the pagecache radix-tree (or page table):
+ * this allows allocators to use a synchronize_rcu() to stabilize _count.
+ *
+ * Unless an RCU grace period has passed, the count of all pages coming out
+ * of the allocator must be considered unstable. page_count may return higher
+ * than expected, and put_page must be able to do the right thing when the
+ * page has been finished with, no matter what it is subsequently allocated
+ * for (because put_page is what is used here to drop an invalid speculative
+ * reference).
+ *
+ * This is the interesting part of the lockless pagecache (and lockless
+ * get_user_pages) locking protocol, where the lookup-side (eg. find_get_page)
+ * has the following pattern:
+ * 1. find page in radix tree
+ * 2. conditionally increment refcount
+ * 3. check the page is still in pagecache (if no, goto 1)
+ *
+ * Remove-side that cares about stability of _count (eg. reclaim) has the
+ * following (with tree_lock held for write):
+ * A. atomically check refcount is correct and set it to 0 (atomic_cmpxchg)
+ * B. remove page from pagecache
+ * C. free the page
+ *
+ * There are 2 critical interleavings that matter:
+ * - 2 runs before A: in this case, A sees elevated refcount and bails out
+ * - A runs before 2: in this case, 2 sees zero refcount and retries;
+ * subsequently, B will complete and 1 will find no page, causing the
+ * lookup to return NULL.
+ *
+ * It is possible that between 1 and 2, the page is removed then the exact same
+ * page is inserted into the same position in pagecache. That's OK: the
+ * old find_get_page using tree_lock could equally have run before or after
+ * such a re-insertion, depending on order that locks are granted.
+ *
+ * Lookups racing against pagecache insertion isn't a big problem: either 1
+ * will find the page or it will not. Likewise, the old find_get_page could run
+ * either before the insertion or afterwards, depending on timing.
+ */
+static inline int page_cache_get_speculative(struct page *page)
+{
+ VM_BUG_ON(in_interrupt());
+
+#ifndef CONFIG_SMP
+# ifdef CONFIG_PREEMPT
+ VM_BUG_ON(!in_atomic());
+# endif
+ /*
+ * Preempt must be disabled here - we rely on rcu_read_lock doing
+ * this for us.
+ *
+ * Pagecache won't be truncated from interrupt context, so if we have
+ * found a page in the radix tree here, we have pinned its refcount by
+ * disabling preempt, and hence no need for the "speculative get" that
+ * SMP requires.
+ */
+ VM_BUG_ON(page_count(page) == 0);
+ atomic_inc(&page->_count);
+
+#else
+ if (unlikely(!get_page_unless_zero(page))) {
+ /*
+ * Either the page has been freed, or will be freed.
+ * In either case, retry here and the caller should
+ * do the right thing (see comments above).
+ */
+ return 0;
+ }
+#endif
+ VM_BUG_ON(PageCompound(page) && (struct page *)page_private(page) != page);
+
+ return 1;
+}
+
+static inline int page_freeze_refs(struct page *page, int count)
+{
+ return likely(atomic_cmpxchg(&page->_count, count, 0) == count);
+}
+
+static inline void page_unfreeze_refs(struct page *page, int count)
+{
+ VM_BUG_ON(page_count(page) != 0);
+ VM_BUG_ON(count == 0);
+
+ atomic_set(&page->_count, count);
+}
+
#ifdef CONFIG_NUMA
extern struct page *__page_cache_alloc(gfp_t gfp);
#else
Index: linux-2.6/mm/vmscan.c
===================================================================
--- linux-2.6.orig/mm/vmscan.c
+++ linux-2.6/mm/vmscan.c
@@ -390,12 +390,10 @@ static pageout_t pageout(struct page *pa
}

/*
- * 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.
+ * Save 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));
@@ -426,9 +424,9 @@ int remove_mapping(struct address_space
* 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();
+ /* note: atomic_cmpxchg in page_freeze_refs provides the smp_rmb */
if (unlikely(PageDirty(page)))
goto cannot_free;

@@ -437,13 +435,11 @@ int remove_mapping(struct address_space
__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:
@@ -452,6 +448,26 @@ 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,
@@ -597,18 +613,27 @@ static unsigned long shrink_page_list(st
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 {
+ nr_reclaimed++;
+ continue;
+ }
+ }
}

- if (!mapping || !remove_mapping(mapping, page))
+ if (!mapping || !__remove_mapping(mapping, page))
goto keep_locked;

free_it:
unlock_page(page);
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:
@@ -622,7 +647,7 @@ 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;
}
Index: linux-2.6/mm/filemap.c
===================================================================
--- linux-2.6.orig/mm/filemap.c
+++ linux-2.6/mm/filemap.c
@@ -466,17 +466,22 @@ int add_to_page_cache(struct page *page,

error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
if (error == 0) {
+ page_cache_get(page);
+ SetPageLocked(page);
+ page->mapping = mapping;
+ page->index = offset;
+
write_lock_irq(&mapping->tree_lock);
error = radix_tree_insert(&mapping->page_tree, offset, page);
- if (!error) {
- page_cache_get(page);
- SetPageLocked(page);
- page->mapping = mapping;
- page->index = offset;
+ if (likely(!error)) {
mapping->nrpages++;
__inc_zone_page_state(page, NR_FILE_PAGES);
- } else
+ } else {
+ page->mapping = NULL;
+ ClearPageLocked(page);
mem_cgroup_uncharge_page(page);
+ page_cache_release(page);
+ }

write_unlock_irq(&mapping->tree_lock);
radix_tree_preload_end();
Index: linux-2.6/mm/swap_state.c
===================================================================
--- linux-2.6.orig/mm/swap_state.c
+++ linux-2.6/mm/swap_state.c
@@ -76,19 +76,26 @@ int add_to_swap_cache(struct page *page,
BUG_ON(PagePrivate(page));
error = radix_tree_preload(gfp_mask);
if (!error) {
+ page_cache_get(page);
+ SetPageSwapCache(page);
+ set_page_private(page, entry.val);
+
write_lock_irq(&swapper_space.tree_lock);
error = radix_tree_insert(&swapper_space.page_tree,
entry.val, page);
- if (!error) {
- page_cache_get(page);
- SetPageSwapCache(page);
- set_page_private(page, entry.val);
+ if (likely(!error)) {
total_swapcache_pages++;
__inc_zone_page_state(page, NR_FILE_PAGES);
INC_CACHE_INFO(add_total);
}
write_unlock_irq(&swapper_space.tree_lock);
radix_tree_preload_end();
+
+ if (unlikely(error)) {
+ set_page_private(page, 0UL);
+ ClearPageSwapCache(page);
+ page_cache_release(page);
+ }
}
return error;
}
Index: linux-2.6/mm/migrate.c
===================================================================
--- linux-2.6.orig/mm/migrate.c
+++ linux-2.6/mm/migrate.c
@@ -304,6 +304,7 @@ out:
static int migrate_page_move_mapping(struct address_space *mapping,
struct page *newpage, struct page *page)
{
+ int expected_count;
void **pslot;

if (!mapping) {
@@ -318,12 +319,18 @@ static int migrate_page_move_mapping(str
pslot = radix_tree_lookup_slot(&mapping->page_tree,
page_index(page));

- if (page_count(page) != 2 + !!PagePrivate(page) ||
+ expected_count = 2 + !!PagePrivate(page);
+ if (page_count(page) != expected_count ||
(struct page *)radix_tree_deref_slot(pslot) != page) {
write_unlock_irq(&mapping->tree_lock);
return -EAGAIN;
}

+ if (!page_freeze_refs(page, expected_count)) {
+ write_unlock_irq(&mapping->tree_lock);
+ return -EAGAIN;
+ }
+
/*
* Now we know that no one else is looking at the page.
*/
@@ -337,6 +344,7 @@ static int migrate_page_move_mapping(str

radix_tree_replace_slot(pslot, newpage);

+ page_unfreeze_refs(page, expected_count);
/*
* Drop cache reference from old page.
* We know this isn't the last reference.
Index: linux-2.6/drivers/net/cassini.c
===================================================================
--- linux-2.6.orig/drivers/net/cassini.c
+++ linux-2.6/drivers/net/cassini.c
@@ -576,6 +576,18 @@ static void cas_spare_recover(struct cas
list_for_each_safe(elem, tmp, &list) {
cas_page_t *page = list_entry(elem, cas_page_t, list);

+ /*
+ * With the lockless pagecache, cassini buffering scheme gets
+ * slightly less accurate: we might find that a page has an
+ * elevated reference count here, due to a speculative ref,
+ * and skip it as in-use. Ideally we would be able to reclaim
+ * it. However this would be such a rare case, it doesn't
+ * matter too much as we should pick it up the next time round.
+ *
+ * Importantly, if we find that the page has a refcount of 1
+ * here (our refcount), then we know it is definitely not inuse
+ * so we can reuse it.
+ */
if (page_count(page->buffer) > 1)
continue;