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charset="utf-8" Content-Transfer-Encoding: base64 Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" T24gVGh1LCBNYXIgMjUsIDIwMjEgYXQgMTA6NTE6MzVBTSArMDEwMCwgVGhvbWFzIEhlbGxzdHLD tm0gKEludGVsKSB3cm90ZToKCj4gPiBQbGVhc2UgZXhwbGFpbiB0aGF0IGZ1cnRoZXIuIFdoeSBk byB3ZSBuZWVkIHRoZSBtbWFwIGxvY2sgdG8gaW5zZXJ0IFBNRHMKPiA+IGJ1dCBub3Qgd2hlbiBp bnNlcnQgUFRFcz8KPiAKPiBXZSBkb24ndC4gQnV0IG9uY2UgeW91J3ZlIGluc2VydGVkIGEgUE1E IGRpcmVjdG9yeSB5b3UgY2FuJ3QgcmVtb3ZlIGl0Cj4gdW5sZXNzIHlvdSBoYXZlIHRoZSBtbWFw IGxvY2sgKGFuZCBwcm9iYWJseSBhbHNvIHRoZSBpX21tYXBfbG9jayBpbiB3cml0ZQo+IG1vZGUp LiBUaGF0IGZvciBleGFtcGxlIG1lYW5zIHRoYXQgaWYgeW91IGhhdmUgYSBWUkFNIHJlZ2lvbiBt YXBwZWQgd2l0aAo+IGh1Z2UgUE1EcywgYW5kIHRoZW4gaXQgZ2V0cyBldmljdGVkLCBhbmQgeW91 IGhhcHBlbiB0byByZWFkIGEgYnl0ZSBmcm9tIGl0Cj4gd2hlbiBpdCdzIGV2aWN0ZWQgYW5kIHRo ZXJlZm9yZSBwb3B1bGF0ZSB0aGUgZnVsbCByZWdpb24gd2l0aCBQVEVzIHBvaW50aW5nCj4gdG8g c3lzdGVtIHBhZ2VzLCB5b3UgY2FuJ3QgZ28gYmFjayB0byBodWdlIFBNRHMgYWdhaW4gd2l0aG91 dCBhIG11bm1hcCgpIGluCj4gYmV0d2Vlbi4KClRoaXMgaXMgYWxsIGJhc2ljYWxseSBtYWdpYyB0 byBtZSBzdGlsbCwgYnV0IFRIUCBkb2VzIHRoaXMKdHJhbnNmb3JtYXRpb24gYW5kIEkgdGhpbmsg d2hhdCBpdCBkb2VzIGNvdWxkIHdvcmsgaGVyZSB0b28uIFdlCnByb2JhYmx5IHdvdWxkbid0IGJl IGFibGUgdG8gdXBncmFkZSB3aGlsZSBoYW5kbGluZyBmYXVsdCwgYnV0IGF0IHRoZQpzYW1lIHRp bWUsIHRoaXMgc2hvdWxkIGJlIHF1aXRlIHJhcmUgYXMgaXQgd291bGQgcmVxdWlyZSB0aGUgZHJp dmVyIHRvCmhhdmUgc3VwcGxpZWQgYSBzbWFsbCBwYWdlIGZvciB0aGlzIFZNQSBhdCBzb21lIHBv aW50LgoKPiA+IEFwYXJ0IGZyb20gdGhhdCBJIHN0aWxsIGRvbid0IGZ1bGx5IGdldCB3aHkgd2Ug bmVlZCB0aGlzIGluIHRoZSBmaXJzdAo+ID4gcGxhY2UuCj4gCj4gQmVjYXVzZSB2aXJ0dWFsIGh1 Z2UgcGFnZSBhZGRyZXNzIGJvdW5kYXJpZXMgbmVlZCB0byBiZSBhbGlnbmVkIHdpdGgKPiBwaHlz aWNhbCBodWdlIHBhZ2UgYWRkcmVzcyBib3VuZGFyaWVzLCBhbmQgbW1hcCBjYW4gaGFwcGVuIGJl Zm9yZSBib3MgYXJlCj4gcG9wdWxhdGVkIHNvIHlvdSBoYXZlIG5vIHdheSBvZiBrbm93aW5nIGhv dyBwaHlzaWNhbCBodWdlIHBhZ2UKPiBhZGRyZXNzCgpCdXQgdGhpcyBpcyBhIG1tYXAtdGltZSBw cm9ibGVtLCBmYXVsdCBjYW4ndCBmaXggbW1hcCB1c2luZyB0aGUgd3JvbmcgVkEuCgo+ID4gSSBy ZWFsbHkgZG9uJ3Qgc2VlIHRoYXQgZWl0aGVyLiBXaGVuIGEgYnVmZmVyIGlzIGFjY2Vzc2VkIGJ5 IHRoZSBDUFUgaXQKPiA+IGlzIGluID4gOTAlIG9mIGFsbCBjYXNlcyBjb21wbGV0ZWx5IGFjY2Vz c2VkLiBOb3QgZmF1bHRpbmcgaW4gZnVsbAo+ID4gcmFuZ2VzIGlzIGp1c3Qgb3B0aW1pemluZyBm b3IgYSByZWFsbHkgdW5saWtlbHkgY2FzZSBoZXJlLgo+IAo+IEl0IG1pZ2h0IGJlIHRoYXQgeW91 J3JlIHJpZ2h0LCBidXQgYXJlIGFsbCBkcml2ZXJzIHdhbnRpbmcgdG8gdXNlIHRoaXMgbGlrZQo+ IGRybSBpbiB0aGlzIHJlc3BlY3Q/IFVzaW5nIHRoZSBpbnRlcmZhY2UgdG8gZmF1bHQgaW4gYSAx RyByYW5nZSBpbiB0aGUgaG9wZQo+IGl0IGNvdWxkIG1hcCBpdCB0byBhIGh1Z2UgcHVkIG1heSB1 bmV4cGVjdGVkbHkgY29uc3VtZSBhbmQgcG9wdWxhdGUgc29tZSAxNisKPiBNQiBvZiBwYWdlIHRh Ymxlcy4KCklmIHRoZSB1bmRlcmx5aW5nIGRldmljZSBibG9jayBzaXplIGlzIHNvIGJpZyB0aGVu IHN1cmUsIHdoeSBub3Q/IFRoZQoidW5leHBlY3RlZGx5IiBzaG91bGQgYmUgcXVpdGUgcmFyZS9u b24gZXhpc3RhbnQgYW55aG93LgoKSmFzb24KIApfX19fX19fX19fX19fX19fX19fX19fX19fX19f X19fX19fX19fX19fX19fX19fXwpkcmktZGV2ZWwgbWFpbGluZyBsaXN0CmRyaS1kZXZlbEBsaXN0 cy5mcmVlZGVza3RvcC5vcmcKaHR0cHM6Ly9saXN0cy5mcmVlZGVza3RvcC5vcmcvbWFpbG1hbi9s aXN0aW5mby9kcmktZGV2ZWwK 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 X-Spam-Level: X-Spam-Status: No, score=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MSGID_FROM_MTA_HEADER,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A8782C433E3 for ; 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Thu, 25 Mar 2021 11:30:25 +0000 Received: from DM6PR12MB3834.namprd12.prod.outlook.com ([fe80::1c62:7fa3:617b:ab87]) by DM6PR12MB3834.namprd12.prod.outlook.com ([fe80::1c62:7fa3:617b:ab87%6]) with mapi id 15.20.3977.029; Thu, 25 Mar 2021 11:30:25 +0000 Date: Thu, 25 Mar 2021 08:30:23 -0300 From: Jason Gunthorpe To: Thomas =?utf-8?B?SGVsbHN0csO2bSAoSW50ZWwp?= Cc: Christian =?utf-8?B?S8O2bmln?= , David Airlie , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-mm@kvack.org, Andrew Morton Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages Message-ID: <20210325113023.GT2356281@nvidia.com> References: <6c9acb90-8e91-d8af-7abd-e762d9a901aa@shipmail.org> <20210324134833.GE2356281@nvidia.com> <0b984f96-00fb-5410-bb16-02e12b2cc024@shipmail.org> <20210324163812.GJ2356281@nvidia.com> <08f19e80-d6cb-8858-0c5d-67d2e2723f72@amd.com> <730eb2ff-ba98-2393-6d42-61735e3c6b83@shipmail.org> <20210324231419.GR2356281@nvidia.com> <607ecbeb-e8a5-66e9-6fe2-9a8d22f12bc2@shipmail.org> <15da5784-96ca-25e5-1485-3ce387ee6695@shipmail.org> Content-Type: text/plain; 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identity=mailfrom; envelope-from=""; helo=NAM10-BN7-obe.outbound.protection.outlook.com; client-ip=40.107.92.71 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616671827-258458 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 Thu, Mar 25, 2021 at 10:51:35AM +0100, Thomas Hellstr=C3=B6m (Intel) w= rote: > > Please explain that further. Why do we need the mmap lock to insert P= MDs > > but not when insert PTEs? >=20 > We don't. But once you've inserted a PMD directory you can't remove it > unless you have the mmap lock (and probably also the i_mmap_lock in wri= te > mode). That for example means that if you have a VRAM region mapped wit= h > huge PMDs, and then it gets evicted, and you happen to read a byte from= it > when it's evicted and therefore populate the full region with PTEs poin= ting > to system pages, you can't go back to huge PMDs again without a munmap(= ) in > between. This is all basically magic to me still, but THP does this transformation and I think what it does could work here too. We probably wouldn't be able to upgrade while handling fault, but at the same time, this should be quite rare as it would require the driver to have supplied a small page for this VMA at some point. > > Apart from that I still don't fully get why we need this in the first > > place. >=20 > Because virtual huge page address boundaries need to be aligned with > physical huge page address boundaries, and mmap can happen before bos a= re > populated so you have no way of knowing how physical huge page > address But this is a mmap-time problem, fault can't fix mmap using the wrong VA. > > I really don't see that either. When a buffer is accessed by the CPU = it > > is in > 90% of all cases completely accessed. Not faulting in full > > ranges is just optimizing for a really unlikely case here. >=20 > It might be that you're right, but are all drivers wanting to use this = like > drm in this respect? Using the interface to fault in a 1G range in the = hope > it could map it to a huge pud may unexpectedly consume and populate som= e 16+ > MB of page tables. If the underlying device block size is so big then sure, why not? The "unexpectedly" should be quite rare/non existant anyhow. Jason =20