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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.

[[email protected]: fix migration_entry_wait()]
[[email protected]: fix add_to_page_cache]
[[email protected]: repair a comment]
Signed-off-by: Nick Piggin <[email protected]>
Cc: Jeff Garzik <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Reviewed-by: Peter Zijlstra <[email protected]>
Signed-off-by: Daisuke Nishimura <[email protected]>
Signed-off-by: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: KOSAKI Motohiro <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Acked-by: Nick Piggin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Nick Piggin 2008-07-25 19:45:30 -07:00 • committed by Linus Torvalds
commit e286781d5f
7 changed files with 227 additions and 45 deletions
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20
mm/migrate.c
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@ -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.