From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B115EC433EF for ; Mon, 4 Oct 2021 14:29:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9191E6136F for ; Mon, 4 Oct 2021 14:29:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234010AbhJDObp (ORCPT ); Mon, 4 Oct 2021 10:31:45 -0400 Received: from outbound-smtp03.blacknight.com ([81.17.249.16]:34025 "EHLO outbound-smtp03.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233767AbhJDObo (ORCPT ); Mon, 4 Oct 2021 10:31:44 -0400 Received: from mail.blacknight.com (pemlinmail02.blacknight.ie [81.17.254.11]) by outbound-smtp03.blacknight.com (Postfix) with ESMTPS id 3ACDAC0BB2 for ; Mon, 4 Oct 2021 15:29:54 +0100 (IST) Received: (qmail 26052 invoked from network); 4 Oct 2021 14:29:54 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.17.29]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 4 Oct 2021 14:29:54 -0000 Date: Mon, 4 Oct 2021 15:29:52 +0100 From: Mel Gorman To: Matthew Wilcox Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC] mm: Optimise put_pages_list() Message-ID: <20211004142952.GN3959@techsingularity.net> References: <20210930163258.3114404-1-willy@infradead.org> <20211004091037.GM3959@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 04, 2021 at 01:49:37PM +0100, Matthew Wilcox wrote: > On Mon, Oct 04, 2021 at 10:10:37AM +0100, Mel Gorman wrote: > > On Thu, Sep 30, 2021 at 05:32:58PM +0100, Matthew Wilcox (Oracle) wrote: > > > Instead of calling put_page() one page at a time, pop pages off > > > the list if there are other refcounts and pass the remainder > > > to free_unref_page_list(). This should be a speed improvement, > > > but I have no measurements to support that. It's also not very > > > widely used today, so I can't say I've really tested it. I'm only > > > bothering with this patch because I'd like the IOMMU code to use it > > > https://lore.kernel.org/lkml/20210930162043.3111119-1-willy@infradead.org/ > > > > > > Signed-off-by: Matthew Wilcox (Oracle) > > > > I see your motivation but you need to check that all users of > > put_pages_list (current and future) handle destroy_compound_page properly > > or handle it within put_pages_list. For example, the release_pages() > > user of free_unref_page_list calls __put_compound_page directly before > > freeing. put_pages_list as it stands will call dstroy_compound_page but > > free_unref_page_list does not destroy compound pages in free_pages_prepare > > Quite right. I was really only thinking about order-zero pages because > there aren't any users of compound pages that call this. But of course, > we should be robust against future callers. So the obvious thing to do > is to copy what release_pages() does: > > +++ b/mm/swap.c > @@ -144,6 +144,10 @@ void put_pages_list(struct list_head *pages) > list_for_each_entry_safe(page, next, pages, lru) { > if (!put_page_testzero(page)) > list_del(&page->lru); > + if (PageCompound(page)) { > + list_del(&page->lru); > + __put_compound_page(page); > + } > } > > free_unref_page_list(pages); That would be the most straight-forward > > But would it be better to have free_unref_page_list() handle compound > pages itself? > > +++ b/mm/page_alloc.c > @@ -3427,6 +3427,11 @@ void free_unref_page_list(struct list_head *list) > > /* Prepare pages for freeing */ > list_for_each_entry_safe(page, next, list, lru) { > + if (PageCompound(page)) { > + __put_compound_page(page); > + list_del(&page->lru); > + continue; > + } > pfn = page_to_pfn(page); > if (!free_unref_page_prepare(page, pfn, 0)) { > list_del(&page->lru); > > (and delete the special handling from release_pages() in the same patch) It's surprisingly tricky. Minimally, that list_del should be before __put_compound_page or you'll clobber whatever list the compound page destructor placed the free page on. Take care with how you remove the special handling and leave a comment explaining why __put_compound_page is not called and that PageLRU will be cleared when it falls through to add the page to pages_to_free. The tricky part is memcg uncharging because if mem_cgroup_uncharge_list() is called then the uncharging happens twice -- once in the destructor and again in mem_cgroup_uncharge_list. I guess you could use two lists and splice them after mem_cgroup_uncharge_list() and before free_unref_page_list. -- Mel Gorman SUSE Labs