mm: page_idle_get_page() does not need lru_lock

It is necessary for page_idle_get_page() to recheck PageLRU() after
get_page_unless_zero(), but holding lru_lock around that serves no
useful purpose, and adds to lru_lock contention: delete it.

See https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop for the
discussion that led to lru_lock there; but __page_set_anon_rmap() now uses
WRITE_ONCE(), and I see no other risk in page_idle_clear_pte_refs() using
rmap_walk() (beyond the risk of racing PageAnon->PageKsm, mostly but not
entirely prevented by page_count() check in ksm.c's write_protect_page():
that risk being shared with page_referenced() and not helped by lru_lock).

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Hugh Dickins <[email protected]>
Signed-off-by: Alex Shi <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: "Huang, Ying" <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Alex Shi <[email protected]>
Cc: Alexander Duyck <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: "Chen, Rong A" <[email protected]>
Cc: Daniel Jordan <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mika Penttilä <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Yang Shi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/page_idle.c

@@ -32,19 +32,15 @@
 static struct page *page_idle_get_page(unsigned long pfn)
 {
 	struct page *page = pfn_to_online_page(pfn);
-	pg_data_t *pgdat;
 
 	if (!page || !PageLRU(page) ||
 	    !get_page_unless_zero(page))
 		return NULL;
 
-	pgdat = page_pgdat(page);
-	spin_lock_irq(&pgdat->lru_lock);
 	if (unlikely(!PageLRU(page))) {
 		put_page(page);
 		page = NULL;
 	}
-	spin_unlock_irq(&pgdat->lru_lock);
 	return page;
 }