[PATCH v20 15/20] mm/lru: introduce TestClearPageLRU

Alex Shi
2020-10-29T10:46:23Z

Currently lru_lock still guards both lru list and page's lru bit, that's
ok. but if we want to use specific lruvec lock on the page, we need to
pin down the page's lruvec/memcg during locking. Just taking lruvec
lock first may be undermined by the page's memcg charge/migration. To
fix this problem, we could clear the lru bit out of locking and use
it as pin down action to block the page isolation in memcg changing.

So now a standard steps of page isolation is following:
1, get_page(); #pin the page avoid to be free
2, TestClearPageLRU(); #block other isolation like memcg change
3, spin_lock on lru_lock; #serialize lru list access
4, delete page from lru list;
The step 2 could be optimzed/replaced in scenarios which page is
unlikely be accessed or be moved between memcgs.

This patch start with the first part: TestClearPageLRU, which combines
PageLRU check and ClearPageLRU into a macro func TestClearPageLRU. This
function will be used as page isolation precondition to prevent other
isolations some where else. Then there are may !PageLRU page on lru
list, need to remove BUG() checking accordingly.

There 2 rules for lru bit now:
1, the lru bit still indicate if a page on lru list, just in some
temporary moment(isolating), the page may have no lru bit when
it's on lru list. but the page still must be on lru list when the
lru bit set.
2, have to remove lru bit before delete it from lru list.

As Andrew Morton mentioned this change would dirty cacheline for page
isn't on LRU. But the lost would be acceptable in Rong Chen
<rong.a.chen@intel.com> report:
https://lore.kernel.org/lkml/20200304090301.GB5972@shao2-debian/

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Cc: cgroups@vger.kernel.org
Cc: linux-mm@kvack.org
---
include/linux/page-flags.h | 1 +
mm/mlock.c | 3 +--
mm/vmscan.c | 39 +++++++++++++++++++--------------------
3 files changed, 21 insertions(+), 22 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 4f6ba9379112..14a0cac9e099 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -335,6 +335,7 @@ static inline void page_init_poison(struct page *page, size_t size)
PAGEFLAG(Dirty, dirty, PF_HEAD) TESTSCFLAG(Dirty, dirty, PF_HEAD)
__CLEARPAGEFLAG(Dirty, dirty, PF_HEAD)
PAGEFLAG(LRU, lru, PF_HEAD) __CLEARPAGEFLAG(LRU, lru, PF_HEAD)
+ TESTCLEARFLAG(LRU, lru, PF_HEAD)
PAGEFLAG(Active, active, PF_HEAD) __CLEARPAGEFLAG(Active, active, PF_HEAD)
TESTCLEARFLAG(Active, active, PF_HEAD)
PAGEFLAG(Workingset, workingset, PF_HEAD)
diff --git a/mm/mlock.c b/mm/mlock.c
index d487aa864e86..7b0e6334be6f 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -276,10 +276,9 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
* We already have pin from follow_page_mask()
* so we can spare the get_page() here.
*/
- if (PageLRU(page)) {
+ if (TestClearPageLRU(page)) {
struct lruvec *lruvec;

- ClearPageLRU(page);
lruvec = mem_cgroup_page_lruvec(page,
page_pgdat(page));
del_page_from_lru_list(page, lruvec,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 845733afccde..ce4ab932805c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1542,7 +1542,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
*/
int __isolate_lru_page(struct page *page, isolate_mode_t mode)
{
- int ret = -EINVAL;
+ int ret = -EBUSY;

/* Only take pages on the LRU. */
if (!PageLRU(page))
@@ -1552,8 +1552,6 @@ int __isolate_lru_page(struct page *page, isolate_mode_t mode)
if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
return ret;

- ret = -EBUSY;
-
/*
* To minimise LRU disruption, the caller can indicate that it only
* wants to isolate pages it will be able to operate on without
@@ -1600,8 +1598,10 @@ int __isolate_lru_page(struct page *page, isolate_mode_t mode)
* sure the page is not being freed elsewhere -- the
* page release code relies on it.
*/
- ClearPageLRU(page);
- ret = 0;
+ if (TestClearPageLRU(page))
+ ret = 0;
+ else
+ put_page(page);
}

return ret;
@@ -1667,8 +1667,6 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
page = lru_to_page(src);
prefetchw_prev_lru_page(page, src, flags);

- VM_BUG_ON_PAGE(!PageLRU(page), page);
-
nr_pages = compound_nr(page);
total_scan += nr_pages;

@@ -1765,21 +1763,18 @@ int isolate_lru_page(struct page *page)
VM_BUG_ON_PAGE(!page_count(page), page);
WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");

- if (PageLRU(page)) {
+ if (TestClearPageLRU(page)) {
pg_data_t *pgdat = page_pgdat(page);
struct lruvec *lruvec;

- spin_lock_irq(&pgdat->lru_lock);
+ get_page(page);
lruvec = mem_cgroup_page_lruvec(page, pgdat);
- if (PageLRU(page)) {
- int lru = page_lru(page);
- get_page(page);
- ClearPageLRU(page);
- del_page_from_lru_list(page, lruvec, lru);
- ret = 0;
- }
+ spin_lock_irq(&pgdat->lru_lock);
+ del_page_from_lru_list(page, lruvec, page_lru(page));
spin_unlock_irq(&pgdat->lru_lock);
+ ret = 0;
}
+
return ret;
}

@@ -4289,6 +4284,10 @@ void check_move_unevictable_pages(struct pagevec *pvec)
nr_pages = thp_nr_pages(page);
pgscanned += nr_pages;

+ /* block memcg migration during page moving between lru */
+ if (!TestClearPageLRU(page))
+ continue;
+
if (pagepgdat != pgdat) {
if (pgdat)
spin_unlock_irq(&pgdat->lru_lock);
@@ -4297,10 +4296,7 @@ void check_move_unevictable_pages(struct pagevec *pvec)
}
lruvec = mem_cgroup_page_lruvec(page, pgdat);

- if (!PageLRU(page) || !PageUnevictable(page))
- continue;
-
- if (page_evictable(page)) {
+ if (page_evictable(page) && PageUnevictable(page)) {
enum lru_list lru = page_lru_base_type(page);

VM_BUG_ON_PAGE(PageActive(page), page);
@@ -4309,12 +4305,15 @@ void check_move_unevictable_pages(struct pagevec *pvec)
add_page_to_lru_list(page, lruvec, lru);
pgrescued += nr_pages;
}
+ SetPageLRU(page);
}

if (pgdat) {
__count_vm_events(UNEVICTABLE_PGRESCUED, pgrescued);
__count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
spin_unlock_irq(&pgdat->lru_lock);
+ } else if (pgscanned) {
+ count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
}
}
EXPORT_SYMBOL_GPL(check_move_unevictable_pages);
--
1.8.3.1