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mm: speculative page references

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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>
This commit is contained in:
Nick Piggin 2008-07-25 19:45:30 -07:00 • committed by Linus Torvalds
parent 47feff2c8e
commit e286781d5f
7 changed files with 227 additions and 45 deletions
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12
drivers/net/cassini.c
View file

@ -576,6 +576,18 @@ static void cas_spare_recover(struct cas *cp, const gfp_t flags)
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;

111
include/linux/pagemap.h
View file

@ -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(struct address_space *m, gfp_t 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());
#if !defined(CONFIG_SMP) && defined(CONFIG_CLASSIC_RCU)
# 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(PageTail(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
@ -133,13 +226,29 @@ static inline struct page *read_mapping_page(struct address_space *mapping,
return read_cache_page(mapping, index, filler, data);
}
int add_to_page_cache(struct page *page, struct address_space *mapping,
int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
pgoff_t index, gfp_t gfp_mask);
int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
pgoff_t index, gfp_t gfp_mask);
extern void remove_from_page_cache(struct page *page);
extern void __remove_from_page_cache(struct page *page);
/*
* Like add_to_page_cache_locked, but used to add newly allocated pages:
* the page is new, so we can just run SetPageLocked() against it.
*/
static inline int add_to_page_cache(struct page *page,
struct address_space *mapping, pgoff_t offset, gfp_t gfp_mask)
{
int error;
SetPageLocked(page);
error = add_to_page_cache_locked(page, mapping, offset, gfp_mask);
if (unlikely(error))
ClearPageLocked(page);
return error;
}
/*
* Return byte-offset into filesystem object for page.
*/

30
mm/filemap.c
View file

@ -442,39 +442,43 @@ int filemap_write_and_wait_range(struct address_space *mapping,
}
/**
* add_to_page_cache - add newly allocated pagecache pages
* add_to_page_cache_locked - add a locked page to the pagecache
* @page: page to add
* @mapping: the page's address_space
* @offset: page index
* @gfp_mask: page allocation mode
*
* This function is used to add newly allocated pagecache pages;
* the page is new, so we can just run SetPageLocked() against it.
* The other page state flags were set by rmqueue().
*
* This function is used to add a page to the pagecache. It must be locked.
* This function does not add the page to the LRU. The caller must do that.
*/
int add_to_page_cache(struct page *page, struct address_space *mapping,
int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
pgoff_t offset, gfp_t gfp_mask)
{
int error = mem_cgroup_cache_charge(page, current->mm,
int error;
VM_BUG_ON(!PageLocked(page));
error = mem_cgroup_cache_charge(page, current->mm,
gfp_mask & ~__GFP_HIGHMEM);
if (error)
goto out;
error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
if (error == 0) {
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;
write_lock_irq(&mapping->tree_lock);
error = radix_tree_insert(&mapping->page_tree, offset, page);
if (likely(!error)) {
mapping->nrpages++;
__inc_zone_page_state(page, NR_FILE_PAGES);
} else
} else {
page->mapping = NULL;
mem_cgroup_uncharge_cache_page(page);
page_cache_release(page);
}
write_unlock_irq(&mapping->tree_lock);
radix_tree_preload_end();
@ -483,7 +487,7 @@ int add_to_page_cache(struct page *page, struct address_space *mapping,
out:
return error;
}
EXPORT_SYMBOL(add_to_page_cache);
EXPORT_SYMBOL(add_to_page_cache_locked);
int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
pgoff_t offset, gfp_t gfp_mask)

20
mm/migrate.c
View file

@ -285,7 +285,15 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
page = migration_entry_to_page(entry);
get_page(page);
/*
* Once radix-tree replacement of page migration started, page_count
* *must* be zero. And, we don't want to call wait_on_page_locked()
* against a page without get_page().
* So, we use get_page_unless_zero(), here. Even failed, page fault
* will occur again.
*/
if (!get_page_unless_zero(page))
goto out;
pte_unmap_unlock(ptep, ptl);
wait_on_page_locked(page);
put_page(page);
@ -305,6 +313,7 @@ out:
static int migrate_page_move_mapping(struct address_space *mapping,
struct page *newpage, struct page *page)
{
int expected_count;
void **pslot;
if (!mapping) {
@ -319,12 +328,18 @@ static int migrate_page_move_mapping(struct address_space *mapping,
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.
*/
@ -338,6 +353,7 @@ static int migrate_page_move_mapping(struct address_space *mapping,
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.

6
mm/shmem.c
View file

@ -936,7 +936,7 @@ found:
spin_lock(&info->lock);
ptr = shmem_swp_entry(info, idx, NULL);
if (ptr && ptr->val == entry.val) {
error = add_to_page_cache(page, inode->i_mapping,
error = add_to_page_cache_locked(page, inode->i_mapping,
idx, GFP_NOWAIT);
/* does mem_cgroup_uncharge_cache_page on error */
} else /* we must compensate for our precharge above */
@ -1301,8 +1301,8 @@ repeat:
SetPageUptodate(filepage);
set_page_dirty(filepage);
swap_free(swap);
} else if (!(error = add_to_page_cache(
swappage, mapping, idx, GFP_NOWAIT))) {
} else if (!(error = add_to_page_cache_locked(swappage, mapping,
idx, GFP_NOWAIT))) {
info->flags |= SHMEM_PAGEIN;
shmem_swp_set(info, entry, 0);
shmem_swp_unmap(entry);

17
mm/swap_state.c
View file

@ -64,7 +64,7 @@ void show_swap_cache_info(void)
}
/*
* add_to_swap_cache resembles add_to_page_cache on swapper_space,
* add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
* but sets SwapCache flag and private instead of mapping and index.
*/
int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
@ -75,20 +75,27 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
BUG_ON(PageSwapCache(page));
BUG_ON(PagePrivate(page));
error = radix_tree_preload(gfp_mask);
if (!error) {
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);
write_lock_irq(&swapper_space.tree_lock);
error = radix_tree_insert(&swapper_space.page_tree,
entry.val, page);
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;
}

72
mm/vmscan.c
View file

@ -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);
__put_page(page);
}
return 1;
cannot_free:
@ -452,6 +450,26 @@ cannot_free:
return 0;
}
/*
* 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
*/
@ -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)
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;
}