From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f200.google.com (mail-pl1-f200.google.com [209.85.214.200]) by kanga.kvack.org (Postfix) with ESMTP id B65BA6B030C for ; Tue, 6 Nov 2018 06:20:38 -0500 (EST) Received: by mail-pl1-f200.google.com with SMTP id s24-v6so12973027plp.12 for ; Tue, 06 Nov 2018 03:20:38 -0800 (PST) Received: from mga18.intel.com (mga18.intel.com. [134.134.136.126]) by mx.google.com with ESMTPS id m3-v6si39265635pld.435.2018.11.06.03.20.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 06 Nov 2018 03:20:37 -0800 (PST) Date: Tue, 6 Nov 2018 19:20:33 +0800 From: Aaron Lu Subject: Re: [PATCH v2 2/2] mm/page_alloc: use a single function to free page Message-ID: <20181106112033.GB24198@intel.com> References: <20181105085820.6341-1-aaron.lu@intel.com> <20181105085820.6341-2-aaron.lu@intel.com> <20181106053037.GD6203@intel.com> <20181106084746.GA24198@intel.com> <30aa9d1f-d619-c143-3de6-6876029538bc@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <30aa9d1f-d619-c143-3de6-6876029538bc@suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, Andrew Morton , =?utf-8?B?UGF3ZcWC?= Staszewski , Jesper Dangaard Brouer , Eric Dumazet , Tariq Toukan , Ilias Apalodimas , Yoel Caspersen , Mel Gorman , Saeed Mahameed , Michal Hocko , Dave Hansen , Alexander Duyck On Tue, Nov 06, 2018 at 10:32:00AM +0100, Vlastimil Babka wrote: > On 11/6/18 9:47 AM, Aaron Lu wrote: > > On Tue, Nov 06, 2018 at 09:16:20AM +0100, Vlastimil Babka wrote: > >> On 11/6/18 6:30 AM, Aaron Lu wrote: > >>> We have multiple places of freeing a page, most of them doing similar > >>> things and a common function can be used to reduce code duplicate. > >>> > >>> It also avoids bug fixed in one function but left in another. > >>> > >>> Signed-off-by: Aaron Lu > >> > >> Acked-by: Vlastimil Babka > > > > Thanks. > > > >> I assume there's no arch that would run page_ref_sub_and_test(1) slower > >> than put_page_testzero(), for the critical __free_pages() case? > > > > Good question. > > > > I followed the non-arch specific calls and found that: > > page_ref_sub_and_test() ends up calling atomic_sub_return(i, v) while > > put_page_testzero() ends up calling atomic_sub_return(1, v). So they > > should be same for archs that do not have their own implementations. > > x86 seems to distinguish between DECL and SUBL, see Ah right. > arch/x86/include/asm/atomic.h although I could not figure out where does > e.g. arch_atomic_dec_and_test become atomic_dec_and_test to override the > generic implementation. I didn't check that either but I think it will :-) > I don't know if the CPU e.g. executes DECL faster, but objectively it > has one parameter less. Maybe it doesn't matter? No immediate idea. > > Back to your question: I don't know either. > > If this is deemed unsafe, we can probably keep the ref modify part in > > their original functions and only take the free part into a common > > function. > > I guess you could also employ if (__builtin_constant_p(nr)) in > free_the_page(), but the result will be ugly I guess, and maybe not > worth it :) Right I can't make it clean. I think I'll just move the free part a common function and leave the ref decreasing part as is to be safe. Regards, Aaron > >>> --- > >>> v2: move comments close to code as suggested by Dave. > >>> > >>> mm/page_alloc.c | 36 ++++++++++++++++-------------------- > >>> 1 file changed, 16 insertions(+), 20 deletions(-) > >>> > >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c > >>> index 91a9a6af41a2..4faf6b7bf225 100644 > >>> --- a/mm/page_alloc.c > >>> +++ b/mm/page_alloc.c > >>> @@ -4425,9 +4425,17 @@ unsigned long get_zeroed_page(gfp_t gfp_mask) > >>> } > >>> EXPORT_SYMBOL(get_zeroed_page); > >>> > >>> -void __free_pages(struct page *page, unsigned int order) > >>> +static inline void free_the_page(struct page *page, unsigned int order, int nr) > >>> { > >>> - if (put_page_testzero(page)) { > >>> + VM_BUG_ON_PAGE(page_ref_count(page) == 0, page); > >>> + > >>> + /* > >>> + * Free a page by reducing its ref count by @nr. > >>> + * If its refcount reaches 0, then according to its order: > >>> + * order0: send to PCP; > >>> + * high order: directly send to Buddy. > >>> + */ > >>> + if (page_ref_sub_and_test(page, nr)) { > >>> if (order == 0) > >>> free_unref_page(page); > >>> else > >>> @@ -4435,6 +4443,10 @@ void __free_pages(struct page *page, unsigned int order) > >>> } > >>> } > >>> > >>> +void __free_pages(struct page *page, unsigned int order) > >>> +{ > >>> + free_the_page(page, order, 1); > >>> +} > >>> EXPORT_SYMBOL(__free_pages); > >>> > >>> void free_pages(unsigned long addr, unsigned int order) > >>> @@ -4481,16 +4493,7 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc, > >>> > >>> void __page_frag_cache_drain(struct page *page, unsigned int count) > >>> { > >>> - VM_BUG_ON_PAGE(page_ref_count(page) == 0, page); > >>> - > >>> - if (page_ref_sub_and_test(page, count)) { > >>> - unsigned int order = compound_order(page); > >>> - > >>> - if (order == 0) > >>> - free_unref_page(page); > >>> - else > >>> - __free_pages_ok(page, order); > >>> - } > >>> + free_the_page(page, compound_order(page), count); > >>> } > >>> EXPORT_SYMBOL(__page_frag_cache_drain); > >>> > >>> @@ -4555,14 +4558,7 @@ void page_frag_free(void *addr) > >>> { > >>> struct page *page = virt_to_head_page(addr); > >>> > >>> - if (unlikely(put_page_testzero(page))) { > >>> - unsigned int order = compound_order(page); > >>> - > >>> - if (order == 0) > >>> - free_unref_page(page); > >>> - else > >>> - __free_pages_ok(page, order); > >>> - } > >>> + free_the_page(page, compound_order(page), 1); > >>> } > >>> EXPORT_SYMBOL(page_frag_free); > >>> > >>> > >> >