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Thu, 23 Jul 2020 13:18:43 +0000 Received: from d06av22.portsmouth.uk.ibm.com (d06av22.portsmouth.uk.ibm.com [9.149.105.58]) by b06cxnps4074.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 06NDIfam61997292 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 23 Jul 2020 13:18:41 GMT Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 668B94C058; Thu, 23 Jul 2020 13:18:41 +0000 (GMT) Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B45894C04E; Thu, 23 Jul 2020 13:18:39 +0000 (GMT) Received: from linux.ibm.com (unknown [9.148.202.80]) by d06av22.portsmouth.uk.ibm.com (Postfix) with ESMTPS; Thu, 23 Jul 2020 13:18:39 +0000 (GMT) Date: Thu, 23 Jul 2020 16:18:37 +0300 From: Mike Rapoport To: Catalin Marinas Subject: Re: =?utf-8?B?562U5aSNOiDnrZTlpI06IFtQQVRD?= =?utf-8?Q?H=5D_arm64=3A_mm=3A_fre?= =?utf-8?Q?e?= unused memmap for sparse memory model that define VMEMMAP Message-ID: <20200723131837.GC1975360@linux.ibm.com> References: <20200721073203.107862-1-liwei213@huawei.com> <20200722060705.GK802087@linux.ibm.com> <1699CE87DE933F49876AD744B5DC140F2312E948@dggemm526-mbx.china.huawei.com> <20200722124910.GE27540@gaia> <1699CE87DE933F49876AD744B5DC140F2312F0D6@dggemm526-mbx.china.huawei.com> <20200723112926.GB7315@gaia> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200723112926.GB7315@gaia> X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235, 18.0.687 definitions=2020-07-23_05:2020-07-23, 2020-07-23 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 adultscore=0 spamscore=0 mlxscore=0 suspectscore=5 mlxlogscore=988 malwarescore=0 lowpriorityscore=0 phishscore=0 bulkscore=0 priorityscore=1501 clxscore=1011 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2007230093 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200723_091948_056848_488D960B X-CRM114-Status: GOOD ( 30.64 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Song Bao Hua \(Barry Song\)" , sujunfei , "linux-arm-kernel@lists.infradead.org" , "steve.capper@arm.com" , "Chenfeng \(puck\)" , "Xiaqing \(A\)" , "linux-kernel@vger.kernel.org" , fengbaopeng , zhaojiapeng , "liwei \(CM\)" , "will@kernel.org" , "nsaenzjulienne@suse.de" , butao Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Jul 23, 2020 at 12:29:26PM +0100, Catalin Marinas wrote: > On Wed, Jul 22, 2020 at 01:40:34PM +0000, liwei (CM) wrote: > > Catalin Marinas wrote: > > > On Wed, Jul 22, 2020 at 08:41:17AM +0000, liwei (CM) wrote: > > > > Mike Rapoport wrote: > > > > > On Tue, Jul 21, 2020 at 03:32:03PM +0800, Wei Li wrote: > > > > > > For the memory hole, sparse memory model that define > > > > > > SPARSEMEM_VMEMMAP do not free the reserved memory for the page > > > > > > map, this patch do it. > > > > > > > > > > Are there numbers showing how much memory is actually freed? > > > > > > > > > > The freeing of empty memmap would become rather complex with these > > > > > changes, do the memory savings justify it? > > > > > > > > In the sparse memory model, the size of a section is 1 GB > > > > (SECTION_SIZE_BITS 30) by default. > > > > > > Can we reduce SECTION_SIZE_BITS instead? Say 26? > > > > Yes, you are right, reduce SECTION_SIZE_BITS to 26 can save almost the > > same memory as the patch. > > > > 1) However, it is not clear whether changing the section size has any > > other impact. > > Well, we should analyse this. > > > 2) Just like the flat memory model and the sparse memory model that > > does not define VMEMMAP, both of them have their own ways to free > > unused memmap. I think we've given a similar way for sparse memory > > define VMEMMAP. > > I think we did it for flatmem initially (on arm32) and added support for > sparsemem later on, so free_unused_memmap() had to cope with sparse > sections. On arm64 we introduced vmemmap support and didn't bother with > the freeing at all because of the added complexity of the vmemmap page > tables. > > I wonder whether we should just disallow flatmem and non-vmemmap > sparsemem on arm64. Is there any value in keeping them around? FLATMEM is useful for UMA systems with a single memory bank, so probably it's worth keeping it for low end machines. Non-vmemmap sparsemem is essentially disable in arch/arm64/Kconfig, so for NUMA configurations SPARSEMEM_VMEMMAP is the only choice. > > 3) This explicit free unused memmap method does reduce unnecessary > > memory waste for users who do not notice the section size > > modification. > > But if we changed SECTION_SIZE_BITS in the mainline kernel, then we > wouldn't need additional code to free the unused memmap. Moreover if we reduce SECTION_SIZE_BITS, we can drop free_unused_memmap() and since the arm64 memory map for sparse will not differ from other arches we can drop custom pfn_valid() as well. > -- > Catalin -- Sincerely yours, Mike. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel