From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from m16.mail.126.com (m16.mail.126.com [117.135.210.8]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 9FA471078B for ; Tue, 18 Jun 2024 03:31:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=117.135.210.8 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718681491; cv=none; b=RaoIaXbiny29qfC+Adyk65yuIwZNATLo2+/gKMRtQOgC9S9bnQasJHxz+7CVFHKE/5EJqjMx5Jp+EW9kWT4R8NOtp7lKTiI+hFPq8p59HrqILEbew9IyALmCo986ju82QotT4Ru2iBFIkw5ibqUnFuTB1jBaJ91jUhG4aCVjNEc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718681491; c=relaxed/simple; bh=SpBt0hbpPezUvX7Dfy11W/uXWq3AonpwV160WsE6UfY=; h=Subject:To:Cc:References:From:Message-ID:Date:MIME-Version: In-Reply-To:Content-Type; b=lBzsNMTd8lH9y+uD6o372BYWOMYXRDd8vcH/Fc1om4lfPFNN0Q/1C2xDsr5TYDJpRBZJC131TBZ6AWe5DG5v2fuf8/UcJbrZob3V5Uj8MzRAE/nwGNBfOxRjLnJR/NNRQODRR/ev73+CFRrJcX46lf5e65QNXrRB/TJ0SPl4JuY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=126.com; spf=pass smtp.mailfrom=126.com; dkim=pass (1024-bit key) header.d=126.com header.i=@126.com header.b=hekzG3fr; arc=none smtp.client-ip=117.135.210.8 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=126.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=126.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=126.com header.i=@126.com header.b="hekzG3fr" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=126.com; s=s110527; h=Subject:From:Message-ID:Date:MIME-Version: Content-Type; bh=FMGSPpXZUiFkhSCU0e4bsz7g6v295g4MMcsy6x3vwJM=; b=hekzG3frY+gSaAFGZg0o8uk4sVJmZ5RAA780VseFoddNY8XMXdmXR5IYYeOaM2 TUTj7SYdmmnH9VzWz//drb+YjNs2oicI4LSrZ+n5Z58FDvhOyZnWuumLUk779uZ+ wcq/nBHwy4OYEc3IDSwC2ui412QUdTH5akfoI2SV3YKB0= Received: from [172.21.21.216] (unknown [118.242.3.34]) by gzga-smtp-mta-g1-5 (Coremail) with SMTP id _____wD3X0x0_3Bm47oVAA--.12793S2; Tue, 18 Jun 2024 11:31:02 +0800 (CST) Subject: Re: [PATCH] mm/page_alloc: skip THP-sized PCP list when allocating non-CMA THP-sized page To: Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, baolin.wang@linux.alibaba.com, liuzixing@hygon.cn References: <1717492460-19457-1-git-send-email-yangge1116@126.com> <2e3a3a3f-737c-ed01-f820-87efee0adc93@126.com> <9b227c9d-f59b-a8b0-b353-7876a56c0bde@126.com> From: yangge1116 Message-ID: <4482bf69-eb07-0ec9-f777-28ce40f96589@126.com> Date: Tue, 18 Jun 2024 11:31:00 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-CM-TRANSID:_____wD3X0x0_3Bm47oVAA--.12793S2 X-Coremail-Antispam: 1Uf129KBjvJXoWxtw17Gr4kuFyxZr4fWFyfJFb_yoWxXr1DpF WfG3W7Ka1UX345Awnrt3Z0kr1Yk343Kr18Wr1rXry8urnIyr1IkF4xJr1UuFyrAry7JF40 qrWUt3ZxZF4UAaDanT9S1TB71UUUUU7qnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDUYxBIdaVFxhVjvjDU0xZFpf9x07bjiSLUUUUU= X-CM-SenderInfo: 51dqwwjhrrila6rslhhfrp/1tbiOhkCG2VEw2HQMwAAsa 在 2024/6/18 上午9:55, Barry Song 写道: > On Tue, Jun 18, 2024 at 9:36 AM yangge1116 wrote: >> >> >> >> 在 2024/6/17 下午8:47, yangge1116 写道: >>> >>> >>> 在 2024/6/17 下午6:26, Barry Song 写道: >>>> On Tue, Jun 4, 2024 at 9:15 PM wrote: >>>>> >>>>> From: yangge >>>>> >>>>> Since commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for >>>>> THP-sized allocations") no longer differentiates the migration type >>>>> of pages in THP-sized PCP list, it's possible to get a CMA page from >>>>> the list, in some cases, it's not acceptable, for example, allocating >>>>> a non-CMA page with PF_MEMALLOC_PIN flag returns a CMA page. >>>>> >>>>> The patch forbids allocating non-CMA THP-sized page from THP-sized >>>>> PCP list to avoid the issue above. >>>> >>>> Could you please describe the impact on users in the commit log? >>> >>> If a large number of CMA memory are configured in the system (for >>> example, the CMA memory accounts for 50% of the system memory), starting >>> virtual machine with device passthrough will get stuck. >>> >>> During starting virtual machine, it will call pin_user_pages_remote(..., >>> FOLL_LONGTERM, ...) to pin memory. If a page is in CMA area, >>> pin_user_pages_remote() will migrate the page from CMA area to non-CMA >>> area because of FOLL_LONGTERM flag. If non-movable allocation requests >>> return CMA memory, pin_user_pages_remote() will enter endless loops. >>> >>> backtrace: >>> pin_user_pages_remote >>> ----__gup_longterm_locked //cause endless loops in this function >>> --------__get_user_pages_locked >>> --------check_and_migrate_movable_pages //always check fail and continue >>> to migrate >>> ------------migrate_longterm_unpinnable_pages >>> ----------------alloc_migration_target // non-movable allocation >>> >>>> Is it possible that some CMA memory might be used by non-movable >>>> allocation requests? >>> >>> Yes. >>> >>> >>>> If so, will CMA somehow become unable to migrate, causing cma_alloc() >>>> to fail? >>> >>> >>> No, it will cause endless loops in __gup_longterm_locked(). If >>> non-movable allocation requests return CMA memory, >>> migrate_longterm_unpinnable_pages() will migrate a CMA page to another >>> CMA page, which is useless and cause endless loops in >>> __gup_longterm_locked(). > > This is only one perspective. We also need to consider the impact on > CMA itself. For example, > when CMA is borrowed by THP, and we need to reclaim it through > cma_alloc() or dma_alloc_coherent(), > we must move those pages out to ensure CMA's users can retrieve that > contiguous memory. > > Currently, CMA's memory is occupied by non-movable pages, meaning we > can't relocate them. > As a result, cma_alloc() is more likely to fail. > >>> >>> backtrace: >>> pin_user_pages_remote >>> ----__gup_longterm_locked //cause endless loops in this function >>> --------__get_user_pages_locked >>> --------check_and_migrate_movable_pages //always check fail and continue >>> to migrate >>> ------------migrate_longterm_unpinnable_pages >>> >>> >>> >>> >>> >>>>> >>>>> Fixes: 5d0a661d808f ("mm/page_alloc: use only one PCP list for >>>>> THP-sized allocations") >>>>> Signed-off-by: yangge >>>>> --- >>>>> mm/page_alloc.c | 10 ++++++++++ >>>>> 1 file changed, 10 insertions(+) >>>>> >>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>>>> index 2e22ce5..0bdf471 100644 >>>>> --- a/mm/page_alloc.c >>>>> +++ b/mm/page_alloc.c >>>>> @@ -2987,10 +2987,20 @@ struct page *rmqueue(struct zone >>>>> *preferred_zone, >>>>> WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1)); >>>>> >>>>> if (likely(pcp_allowed_order(order))) { >>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE >>>>> + if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & >>>>> ALLOC_CMA || >>>>> + order != >>>>> HPAGE_PMD_ORDER) { >>>>> + page = rmqueue_pcplist(preferred_zone, zone, >>>>> order, >>>>> + migratetype, >>>>> alloc_flags); >>>>> + if (likely(page)) >>>>> + goto out; >>>>> + } >>>> >>>> This seems not ideal, because non-CMA THP gets no chance to use PCP. >>>> But it >>>> still seems better than causing the failure of CMA allocation. >>>> >>>> Is there a possible approach to avoiding adding CMA THP into pcp from >>>> the first >>>> beginning? Otherwise, we might need a separate PCP for CMA. >>>> >> >> The vast majority of THP-sized allocations are GFP_MOVABLE, avoiding >> adding CMA THP into pcp may incur a slight performance penalty. >> > > But the majority of movable pages aren't CMA, right? > Do we have an estimate for > adding back a CMA THP PCP? Will per_cpu_pages introduce a new cacheline, which > the original intention for THP was to avoid by having only one PCP[1]? > > [1] https://patchwork.kernel.org/project/linux-mm/patch/20220624125423.6126-3-mgorman@techsingularity.net/ > The size of struct per_cpu_pages is 256 bytes in current code containing commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for THP-sized allocations"). crash> struct per_cpu_pages struct per_cpu_pages { spinlock_t lock; int count; int high; int high_min; int high_max; int batch; u8 flags; u8 alloc_factor; u8 expire; short free_count; struct list_head lists[13]; } SIZE: 256 After revert commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for THP-sized allocations"), the size of struct per_cpu_pages is 272 bytes. crash> struct per_cpu_pages struct per_cpu_pages { spinlock_t lock; int count; int high; int high_min; int high_max; int batch; u8 flags; u8 alloc_factor; u8 expire; short free_count; struct list_head lists[15]; } SIZE: 272 Seems commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for THP-sized allocations") decrease one cacheline. > >> Commit 1d91df85f399 takes a similar approach to filter, and I mainly >> refer to it. >> >> >>>>> +#else >>>>> page = rmqueue_pcplist(preferred_zone, zone, order, >>>>> migratetype, alloc_flags); >>>>> if (likely(page)) >>>>> goto out; >>>>> +#endif >>>>> } >>>>> >>>>> page = rmqueue_buddy(preferred_zone, zone, order, alloc_flags, >>>>> -- >>>>> 2.7.4 >>>> >>>> Thanks >>>> Barry >>>> >> >>