mm/lru: revise the comments of lru_lock

Since we changed the pgdat->lru_lock to lruvec->lru_lock, it's time to fix
the incorrect comments in code.  Also fixed some zone->lru_lock comment
error from ancient time.  etc.

I struggled to understand the comment above move_pages_to_lru() (surely
it never calls page_referenced()), and eventually realized that most of
it had got separated from shrink_active_list(): move that comment back.

Link: https://lkml.kernel.org/r/1604566549-62481-20-git-send-email-alex.shi@linux.alibaba.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Jann Horn <jannh@google.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Alexander Duyck <alexander.duyck@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Chen, Rong A" <rong.a.chen@intel.com>
Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mika Penttilä <mika.penttila@nextfour.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 23 insertions, 18 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 60705ea598ee..257cba79a96d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1613,14 +1613,16 @@ static __always_inline void update_lru_sizes(struct lruvec *lruvec,
 }
 
 /**
- * pgdat->lru_lock is heavily contended.  Some of the functions that
+ * Isolating page from the lruvec to fill in @dst list by nr_to_scan times.
+ *
+ * lruvec->lru_lock is heavily contended.  Some of the functions that
  * shrink the lists perform better by taking out a batch of pages
  * and working on them outside the LRU lock.
  *
  * For pagecache intensive workloads, this function is the hottest
  * spot in the kernel (apart from copy_*_user functions).
  *
- * Appropriate locks must be held before calling this function.
+ * Lru_lock must be held before calling this function.
  *
  * @nr_to_scan:	The number of eligible pages to look through on the list.
  * @lruvec:	The LRU vector to pull pages from.
@@ -1814,25 +1816,11 @@ static int too_many_isolated(struct pglist_data *pgdat, int file,
 }
 
 /*
- * This moves pages from @list to corresponding LRU list.
- *
- * We move them the other way if the page is referenced by one or more
- * processes, from rmap.
- *
- * If the pages are mostly unmapped, the processing is fast and it is
- * appropriate to hold zone_lru_lock across the whole operation.  But if
- * the pages are mapped, the processing is slow (page_referenced()) so we
- * should drop zone_lru_lock around each page.  It's impossible to balance
- * this, so instead we remove the pages from the LRU while processing them.
- * It is safe to rely on PG_active against the non-LRU pages in here because
- * nobody will play with that bit on a non-LRU page.
- *
- * The downside is that we have to touch page->_refcount against each page.
- * But we had to alter page->flags anyway.
+ * move_pages_to_lru() moves pages from private @list to appropriate LRU list.
+ * On return, @list is reused as a list of pages to be freed by the caller.
  *
  * Returns the number of pages moved to the given lruvec.
  */
-
 static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
 						     struct list_head *list)
 {
@@ -2010,6 +1998,23 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 	return nr_reclaimed;
 }
 
+/*
+ * shrink_active_list() moves pages from the active LRU to the inactive LRU.
+ *
+ * We move them the other way if the page is referenced by one or more
+ * processes.
+ *
+ * If the pages are mostly unmapped, the processing is fast and it is
+ * appropriate to hold lru_lock across the whole operation.  But if
+ * the pages are mapped, the processing is slow (page_referenced()), so
+ * we should drop lru_lock around each page.  It's impossible to balance
+ * this, so instead we remove the pages from the LRU while processing them.
+ * It is safe to rely on PG_active against the non-LRU pages in here because
+ * nobody will play with that bit on a non-LRU page.
+ *
+ * The downside is that we have to touch page->_refcount against each page.
+ * But we had to alter page->flags anyway.
+ */
 static void shrink_active_list(unsigned long nr_to_scan,
 			       struct lruvec *lruvec,
 			       struct scan_control *sc,