The patch titled
Subject: mm, mlock, vmscan: no more skipping pagevecs
has been added to the -mm tree. Its filename is
mm-mlock-vmscan-no-more-skipping-pagevecs.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-mlock-vmscan-no-more-skipping-pagevecs.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-mlock-vmscan-no-more-skipping-pagevecs.patch
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From: Shakeel Butt <shakeelb@xxxxxxxxxx>
Subject: mm, mlock, vmscan: no more skipping pagevecs
When a thread mlocks an address space backed either by file pages which
are currently not present in memory or swapped out anon pages (not in
swapcache), a new page is allocated and added to the local pagevec
(lru_add_pvec), I/O is triggered and the thread then sleeps on the page.
On I/O completion, the thread can wake on a different CPU, the mlock
syscall will then sets the PageMlocked() bit of the page but will not be
able to put that page in unevictable LRU as the page is on the pagevec of
a different CPU. Even on drain, that page will go to evictable LRU
because the PageMlocked() bit is not checked on pagevec drain.
The page will eventually go to right LRU on reclaim but the LRU stats will
remain skewed for a long time.
This patch puts all the pages, even unevictable, to the pagevecs and on
the drain, the pages will be added on their LRUs correctly by checking
their evictability. This resolves the mlocked pages on pagevec of other
CPUs issue because when those pagevecs will be drained, the mlocked file
pages will go to unevictable LRU. Also this makes the race with munlock
easier to resolve because the pagevec drains happen in LRU lock.
However there is still one place which makes a page evictable and does
PageLRU check on that page without LRU lock and needs special attention.
TestClearPageMlocked() and isolate_lru_page() in clear_page_mlock().
#0: __pagevec_lru_add_fn #1: clear_page_mlock
SetPageLRU() if (!TestClearPageMlocked())
return
smp_mb() // <--required
// inside does PageLRU
if (!PageMlocked()) if (isolate_lru_page())
move to evictable LRU putback_lru_page()
else
move to unevictable LRU
In '#1', TestClearPageMlocked() provides full memory barrier semantics and
thus the PageLRU check (inside isolate_lru_page) can not be reordered
before it.
In '#0', without explicit memory barrier, the PageMlocked() check can be
reordered before SetPageLRU(). If that happens, '#0' can put a page in
unevictable LRU and '#1' might have just cleared the Mlocked bit of that
page but fails to isolate as PageLRU fails as '#0' still hasn't set
PageLRU bit of that page. That page will be stranded on the unevictable
LRU.
There is one (good) side effect though. Without this patch, the pages
allocated for System V shared memory segment are added to evictable LRUs
even after shmctl(SHM_LOCK) on that segment. This patch will correctly
put such pages to unevictable LRU.
Link: http://lkml.kernel.org/r/20171121211241.18877-1-shakeelb@xxxxxxxxxx
Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Greg Thelen <gthelen@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Balbir Singh <bsingharora@xxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Shaohua Li <shli@xxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/swap.h | 2 -
mm/mlock.c | 6 +++
mm/swap.c | 82 +++++++++++++++++++++++------------------
mm/vmscan.c | 59 -----------------------------
4 files changed, 54 insertions(+), 95 deletions(-)
diff -puN include/linux/swap.h~mm-mlock-vmscan-no-more-skipping-pagevecs include/linux/swap.h
--- a/include/linux/swap.h~mm-mlock-vmscan-no-more-skipping-pagevecs
+++ a/include/linux/swap.h
@@ -337,8 +337,6 @@ extern void deactivate_file_page(struct
extern void mark_page_lazyfree(struct page *page);
extern void swap_setup(void);
-extern void add_page_to_unevictable_list(struct page *page);
-
extern void lru_cache_add_active_or_unevictable(struct page *page,
struct vm_area_struct *vma);
diff -puN mm/mlock.c~mm-mlock-vmscan-no-more-skipping-pagevecs mm/mlock.c
--- a/mm/mlock.c~mm-mlock-vmscan-no-more-skipping-pagevecs
+++ a/mm/mlock.c
@@ -64,6 +64,12 @@ void clear_page_mlock(struct page *page)
mod_zone_page_state(page_zone(page), NR_MLOCK,
-hpage_nr_pages(page));
count_vm_event(UNEVICTABLE_PGCLEARED);
+ /*
+ * The previous TestClearPageMlocked() corresponds to the smp_mb()
+ * in __pagevec_lru_add_fn().
+ *
+ * See __pagevec_lru_add_fn for more explanation.
+ */
if (!isolate_lru_page(page)) {
putback_lru_page(page);
} else {
diff -puN mm/swap.c~mm-mlock-vmscan-no-more-skipping-pagevecs mm/swap.c
--- a/mm/swap.c~mm-mlock-vmscan-no-more-skipping-pagevecs
+++ a/mm/swap.c
@@ -446,30 +446,6 @@ void lru_cache_add(struct page *page)
}
/**
- * add_page_to_unevictable_list - add a page to the unevictable list
- * @page: the page to be added to the unevictable list
- *
- * Add page directly to its zone's unevictable list. To avoid races with
- * tasks that might be making the page evictable, through eg. munlock,
- * munmap or exit, while it's not on the lru, we want to add the page
- * while it's locked or otherwise "invisible" to other tasks. This is
- * difficult to do when using the pagevec cache, so bypass that.
- */
-void add_page_to_unevictable_list(struct page *page)
-{
- struct pglist_data *pgdat = page_pgdat(page);
- struct lruvec *lruvec;
-
- spin_lock_irq(&pgdat->lru_lock);
- lruvec = mem_cgroup_page_lruvec(page, pgdat);
- ClearPageActive(page);
- SetPageUnevictable(page);
- SetPageLRU(page);
- add_page_to_lru_list(page, lruvec, LRU_UNEVICTABLE);
- spin_unlock_irq(&pgdat->lru_lock);
-}
-
-/**
* lru_cache_add_active_or_unevictable
* @page: the page to be added to LRU
* @vma: vma in which page is mapped for determining reclaimability
@@ -484,13 +460,9 @@ void lru_cache_add_active_or_unevictable
{
VM_BUG_ON_PAGE(PageLRU(page), page);
- if (likely((vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) != VM_LOCKED)) {
+ if (likely((vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) != VM_LOCKED))
SetPageActive(page);
- lru_cache_add(page);
- return;
- }
-
- if (!TestSetPageMlocked(page)) {
+ else if (!TestSetPageMlocked(page)) {
/*
* We use the irq-unsafe __mod_zone_page_stat because this
* counter is not modified from interrupt context, and the pte
@@ -500,7 +472,7 @@ void lru_cache_add_active_or_unevictable
hpage_nr_pages(page));
count_vm_event(UNEVICTABLE_PGMLOCKED);
}
- add_page_to_unevictable_list(page);
+ lru_cache_add(page);
}
/*
@@ -886,15 +858,55 @@ void lru_add_page_tail(struct page *page
static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec,
void *arg)
{
- int file = page_is_file_cache(page);
- int active = PageActive(page);
- enum lru_list lru = page_lru(page);
+ enum lru_list lru;
+ int was_unevictable = TestClearPageUnevictable(page);
VM_BUG_ON_PAGE(PageLRU(page), page);
SetPageLRU(page);
+ /*
+ * Page becomes evictable in two ways:
+ * 1) Within LRU lock [munlock_vma_pages() and __munlock_pagevec()].
+ * 2) Before acquiring LRU lock to put the page to correct LRU and then
+ * a) do PageLRU check with lock [check_move_unevictable_pages]
+ * b) do PageLRU check before lock [clear_page_mlock]
+ *
+ * (1) & (2a) are ok as LRU lock will serialize them. For (2b), we need
+ * following strict ordering:
+ *
+ * #0: __pagevec_lru_add_fn #1: clear_page_mlock
+ *
+ * SetPageLRU() TestClearPageMlocked()
+ * smp_mb() // explicit ordering // above provides strict
+ * // ordering
+ * PageMlocked() PageLRU()
+ *
+ *
+ * if '#1' does not observe setting of PG_lru by '#0' and fails
+ * isolation, the explicit barrier will make sure that page_evictable
+ * check will put the page in correct LRU. Without smp_mb(), SetPageLRU
+ * can be reordered after PageMlocked check and can make '#1' to fail
+ * the isolation of the page whose Mlocked bit is cleared (#0 is also
+ * looking at the same page) and the evictable page will be stranded
+ * in an unevictable LRU.
+ */
+ smp_mb();
+
+ if (page_evictable(page)) {
+ lru = page_lru(page);
+ update_page_reclaim_stat(lruvec, page_is_file_cache(page),
+ PageActive(page));
+ if (was_unevictable)
+ count_vm_event(UNEVICTABLE_PGRESCUED);
+ } else {
+ lru = LRU_UNEVICTABLE;
+ ClearPageActive(page);
+ SetPageUnevictable(page);
+ if (!was_unevictable)
+ count_vm_event(UNEVICTABLE_PGCULLED);
+ }
+
add_page_to_lru_list(page, lruvec, lru);
- update_page_reclaim_stat(lruvec, file, active);
trace_mm_lru_insertion(page, lru);
}
diff -puN mm/vmscan.c~mm-mlock-vmscan-no-more-skipping-pagevecs mm/vmscan.c
--- a/mm/vmscan.c~mm-mlock-vmscan-no-more-skipping-pagevecs
+++ a/mm/vmscan.c
@@ -773,64 +773,7 @@ int remove_mapping(struct address_space
*/
void putback_lru_page(struct page *page)
{
- bool is_unevictable;
- int was_unevictable = PageUnevictable(page);
-
- VM_BUG_ON_PAGE(PageLRU(page), page);
-
-redo:
- ClearPageUnevictable(page);
-
- if (page_evictable(page)) {
- /*
- * For evictable pages, we can use the cache.
- * In event of a race, worst case is we end up with an
- * unevictable page on [in]active list.
- * We know how to handle that.
- */
- is_unevictable = false;
- lru_cache_add(page);
- } else {
- /*
- * Put unevictable pages directly on zone's unevictable
- * list.
- */
- is_unevictable = true;
- add_page_to_unevictable_list(page);
- /*
- * When racing with an mlock or AS_UNEVICTABLE clearing
- * (page is unlocked) make sure that if the other thread
- * does not observe our setting of PG_lru and fails
- * isolation/check_move_unevictable_pages,
- * we see PG_mlocked/AS_UNEVICTABLE cleared below and move
- * the page back to the evictable list.
- *
- * The other side is TestClearPageMlocked() or shmem_lock().
- */
- smp_mb();
- }
-
- /*
- * page's status can change while we move it among lru. If an evictable
- * page is on unevictable list, it never be freed. To avoid that,
- * check after we added it to the list, again.
- */
- if (is_unevictable && page_evictable(page)) {
- if (!isolate_lru_page(page)) {
- put_page(page);
- goto redo;
- }
- /* This means someone else dropped this page from LRU
- * So, it will be freed or putback to LRU again. There is
- * nothing to do here.
- */
- }
-
- if (was_unevictable && !is_unevictable)
- count_vm_event(UNEVICTABLE_PGRESCUED);
- else if (!was_unevictable && is_unevictable)
- count_vm_event(UNEVICTABLE_PGCULLED);
-
+ lru_cache_add(page);
put_page(page); /* drop ref from isolate */
}
_
Patches currently in -mm which might be from shakeelb@xxxxxxxxxx are
mm-mlock-vmscan-no-more-skipping-pagevecs.patch
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