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Wed, 11 Nov 2020 12:52:06 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id B415FABDE; Wed, 11 Nov 2020 12:52:05 +0000 (UTC) Subject: Re: [PATCH v21 07/19] mm: page_idle_get_page() does not need lru_lock To: huang ying , Alex Shi Cc: Andrew Morton , Mel Gorman , Tejun Heo , Hugh Dickins , khlebnikov@yandex-team.ru, daniel.m.jordan@oracle.com, willy@infradead.org, Johannes Weiner , lkp@intel.com, linux-mm@kvack.org, LKML , cgroups@vger.kernel.org, Shakeel Butt , iamjoonsoo.kim@lge.com, richard.weiyang@gmail.com, kirill@shutemov.name, alexander.duyck@gmail.com, kernel test robot , Michal Hocko , Vladimir Davydov , shy828301@gmail.com, Minchan Kim References: <1604566549-62481-1-git-send-email-alex.shi@linux.alibaba.com> <1604566549-62481-8-git-send-email-alex.shi@linux.alibaba.com> From: Vlastimil Babka Message-ID: Date: Wed, 11 Nov 2020 13:52:01 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 11/11/20 9:17 AM, huang ying wrote: > On Thu, Nov 5, 2020 at 4:56 PM Alex Shi wrote: >> >> From: Hugh Dickins >> >> 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). >> >> Signed-off-by: Hugh Dickins >> Signed-off-by: Alex Shi >> Cc: Andrew Morton >> Cc: Vladimir Davydov >> Cc: Vlastimil Babka >> Cc: Minchan Kim >> Cc: Alex Shi >> Cc: linux-mm@kvack.org >> Cc: linux-kernel@vger.kernel.org >> --- >> mm/page_idle.c | 4 ---- >> 1 file changed, 4 deletions(-) >> >> diff --git a/mm/page_idle.c b/mm/page_idle.c >> index 057c61df12db..64e5344a992c 100644 >> --- a/mm/page_idle.c >> +++ b/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); > > get_page_unless_zero() is a full memory barrier. But do we need a > compiler barrier here to prevent the compiler to cache PageLRU() > results here? Otherwise looks OK to me, I think the compiler barrier is also implied by the full memory barrier and prevents the caching. Acked-by: Vlastimil Babka > Acked-by: "Huang, Ying" > > Best Regards, > Huang, Ying > >> if (unlikely(!PageLRU(page))) { >> put_page(page); >> page = NULL; >> } >> - spin_unlock_irq(&pgdat->lru_lock); >> return page; >> } >> >> -- >> 1.8.3.1 >> >> >