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Tue, 18 May 2021 19:56:22 +0000 Subject: Re: [PATCH v1 04/11] mm/memremap: add ZONE_DEVICE support for compound pages To: Joao Martins , Dan Williams Cc: Linux MM , Ira Weiny , linux-nvdimm , Matthew Wilcox , Jason Gunthorpe , Muchun Song , Mike Kravetz , Andrew Morton References: <20210325230938.30752-1-joao.m.martins@oracle.com> <20210325230938.30752-5-joao.m.martins@oracle.com> <56a3e271-4ef8-ba02-639e-fd7fe7de7e36@oracle.com> <8c922a58-c901-1ad9-5d19-1182bd6dea1e@oracle.com> From: Jane Chu Organization: Oracle Corporation Message-ID: Date: Tue, 18 May 2021 12:56:18 -0700 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.9.1 In-Reply-To: <8c922a58-c901-1ad9-5d19-1182bd6dea1e@oracle.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Originating-IP: [108.226.113.12] X-ClientProxiedBy: SN1PR12CA0052.namprd12.prod.outlook.com (2603:10b6:802:20::23) To SJ0PR10MB4429.namprd10.prod.outlook.com (2603:10b6:a03:2d1::14) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.1.70] (108.226.113.12) by SN1PR12CA0052.namprd12.prod.outlook.com (2603:10b6:802:20::23) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4108.25 via Frontend Transport; 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dkim=pass header.d=oracle.com header.s=corp-2020-01-29 header.b=KWbrXa02; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b=I55pI0b7; dmarc=pass (policy=none) header.from=oracle.com; spf=none (imf21.hostedemail.com: domain of jane.chu@oracle.com has no SPF policy when checking 156.151.31.85) smtp.mailfrom=jane.chu@oracle.com X-Stat-Signature: grsw988yowidtox665m13u9hcb4arz8r X-Rspamd-Queue-Id: 8AF11E0001B4 X-Rspamd-Server: rspam02 X-HE-Tag: 1621367813-875178 Content-Transfer-Encoding: quoted-printable 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 5/18/2021 10:27 AM, Joao Martins wrote: > On 5/5/21 11:36 PM, Joao Martins wrote: >> On 5/5/21 11:20 PM, Dan Williams wrote: >>> On Wed, May 5, 2021 at 12:50 PM Joao Martins wrote: >>>> On 5/5/21 7:44 PM, Dan Williams wrote: >>>>> On Thu, Mar 25, 2021 at 4:10 PM Joao Martins wrote: >>>>>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h >>>>>> index b46f63dcaed3..bb28d82dda5e 100644 >>>>>> --- a/include/linux/memremap.h >>>>>> +++ b/include/linux/memremap.h >>>>>> @@ -114,6 +114,7 @@ struct dev_pagemap { >>>>>> struct completion done; >>>>>> enum memory_type type; >>>>>> unsigned int flags; >>>>>> + unsigned long align; >>>>> I think this wants some kernel-doc above to indicate that non-zero >>>>> means "use compound pages with tail-page dedup" and zero / PAGE_SIZ= E >>>>> means "use non-compound base pages". > [...] > >>>>> The non-zero value must be >>>>> PAGE_SIZE, PMD_PAGE_SIZE or PUD_PAGE_SIZE. >>>>> Hmm, maybe it should be an >>>>> enum: >>>>> >>>>> enum devmap_geometry { >>>>> DEVMAP_PTE, >>>>> DEVMAP_PMD, >>>>> DEVMAP_PUD, >>>>> } >>>>> >>>> I suppose a converter between devmap_geometry and page_size would be= needed too? And maybe >>>> the whole dax/nvdimm align values change meanwhile (as a followup im= provement)? >>> I think it is ok for dax/nvdimm to continue to maintain their align >>> value because it should be ok to have 4MB align if the device really >>> wanted. However, when it goes to map that alignment with >>> memremap_pages() it can pick a mode. For example, it's already the >>> case that dax->align =3D=3D 1GB is mapped with DEVMAP_PTE today, so >>> they're already separate concepts that can stay separate. >>> >> Gotcha. > I am reconsidering part of the above. In general, yes, the meaning of d= evmap @align > represents a slightly different variation of the device @align i.e. how= the metadata is > laid out **but** regardless of what kind of page table entries we use v= memmap. > > By using DEVMAP_PTE/PMD/PUD we might end up 1) duplicating what nvdimm/= dax already > validates in terms of allowed device @align values (i.e. PAGE_SIZE, PMD= _SIZE and PUD_SIZE) > 2) the geometry of metadata is very much tied to the value we pick to @= align at namespace > provisioning -- not the "align" we might use at mmap() perhaps that's w= hat you referred > above? -- and 3) the value of geometry actually derives from dax device= @align because we > will need to create compound pages representing a page size of @align v= alue. > > Using your example above: you're saying that dax->align =3D=3D 1G is ma= pped with DEVMAP_PTEs, > in reality the vmemmap is populated with PMDs/PUDs page tables (dependi= ng on what archs > decide to do at vmemmap_populate()) and uses base pages as its metadata= regardless of what > device @align. In reality what we want to convey in @geometry is not pa= ge table sizes, but > just the page size used for the vmemmap of the dax device. Additionally= , limiting its > value might not be desirable... if tomorrow Linux for some arch support= s dax/nvdimm > devices with 4M align or 64K align, the value of @geometry will have to= reflect the 4M to > create compound pages of order 10 for the said vmemmap. > > I am going to wait until you finish reviewing the remaining four patche= s of this series, > but maybe this is a simple misnomer (s/align/geometry/) with a comment = but without > DEVMAP_{PTE,PMD,PUD} enum part? Or perhaps its own struct with a value = and enum a > setter/getter to audit its value? Thoughts? > > Joao Good points there. My understanding is that=C2=A0 dax->align=C2=A0 conveys granularity of si= ze while=20 carving out a namespace, it's a geometry attribute loosely akin to sector size of a spindle=20 disk.=C2=A0 I tend to think that device pagesize=C2=A0 has almost no relation to "align" in that, it's=20 possible to have 1G "align" and 4K pagesize, or verse versa.=C2=A0 That is, with the advent of compound p= age=20 support, it is possible to totally separate the two concepts. How about adding a new option to "ndctl create-namespace" that describes=20 device creator's desired pagesize, and another parameter to describe whether the pagesize shall=20 be fixed or allowed to be split up, such that, if the intention is to never split up 2M pagesize, then it=20 would be possible to save a lot metadata space on the device? thanks, -jane