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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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74
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);
}
__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)
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;
}