[patch 07/19] mm: page_idle_get_page() does not need lru_lock

From: Andrew Morton <akpm@linux-foundation.org>
To: aarcange@redhat.com, akpm@linux-foundation.org,
	alex.shi@linux.alibaba.com, alexander.duyck@gmail.com,
	aryabinin@virtuozzo.com, daniel.m.jordan@oracle.com,
	hannes@cmpxchg.org, hughd@google.com, iamjoonsoo.kim@lge.com,
	jannh@google.com, khlebnikov@yandex-team.ru,
	kirill.shutemov@linux.intel.com, kirill@shutemov.name,
	linux-mm@kvack.org, mgorman@techsingularity.net,
	mhocko@kernel.org, mhocko@suse.com, mika.penttila@nextfour.com,
	minchan@kernel.org, mm-commits@vger.kernel.org,
	richard.weiyang@gmail.com, rong.a.chen@intel.com,
	shakeelb@google.com, tglx@linutronix.de, tj@kernel.org,
	torvalds@linux-foundation.org, vbabka@suse.cz,
	vdavydov.dev@gmail.com, willy@infradead.org,
	yang.shi@linux.alibaba.com, ying.huang@intel.com
Subject: [patch 07/19] mm: page_idle_get_page() does not need lru_lock
Date: Tue, 15 Dec 2020 12:33:47 -0800	[thread overview]
Message-ID: <20201215203347.kUAcWnZn0%akpm@linux-foundation.org> (raw)
In-Reply-To: <20201215123253.954eca9a5ef4c0d52fd381fa@linux-foundation.org>

From: Hugh Dickins <hughd@google.com>
Subject: 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/1604566549-62481-8-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: "Huang, Ying" <ying.huang@intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: "Chen, Rong A" <rong.a.chen@intel.com>
Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Jann Horn <jannh@google.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: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mika Penttilä <mika.penttila@nextfour.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_idle.c |    4 ----
 1 file changed, 4 deletions(-)

--- a/mm/page_idle.c~mm-page_idle_get_page-does-not-need-lru_lock
+++ a/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;
 }
 
_
