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Wed, 24 Mar 2021 14:35:43 +0100 (CET) Authentication-Results: pio-pvt-msa1.bahnhof.se; dkim=pass (1024-bit key; unprotected) header.d=shipmail.org header.i=@shipmail.org header.b="jLDwV+Er"; dkim-atps=neutral X-Virus-Scanned: Debian amavisd-new at bahnhof.se Received: from pio-pvt-msa1.bahnhof.se ([127.0.0.1]) by localhost (pio-pvt-msa1.bahnhof.se [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id JCCawRD9HjzC; Wed, 24 Mar 2021 14:35:42 +0100 (CET) Received: by pio-pvt-msa1.bahnhof.se (Postfix) with ESMTPA id 5BC633F240; Wed, 24 Mar 2021 14:35:41 +0100 (CET) Received: from [10.249.254.166] (unknown [192.198.151.44]) by mail1.shipmail.org (Postfix) with ESMTPSA id 1094F3605CC; Wed, 24 Mar 2021 14:35:39 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=shipmail.org; s=mail; t=1616592940; bh=xXXPaXg/ez/sdT5QhHcHgxfrUBmPliIL2wIReLDhQjg=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=jLDwV+ErQCFc9TDEXXtEKVz3UmVZeq8QhPHnWwi1TirvzsnfI9q3YDlRhzlQo9Jdb rk12O2Ix72T/YIQT+f5qDz1zPAUTO3Clcf4MPsp8s7O6hWzwRbI+Kta7xrDj1SWjL8 yytllN4Da6Ny67AVboYIif4JsFGznBS98kkmWoeY= Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages To: Jason Gunthorpe Cc: David Airlie , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-mm@kvack.org, Andrew Morton , Christian Koenig References: <20210321184529.59006-1-thomas_os@shipmail.org> <20210321184529.59006-2-thomas_os@shipmail.org> <314fc020-d243-dbf0-acb3-ecfcc9c2443c@shipmail.org> <20210323163715.GJ2356281@nvidia.com> <5824b731-ca6a-92fd-e314-d986b6a7b101@shipmail.org> <20210324122430.GW2356281@nvidia.com> <20210324124127.GY2356281@nvidia.com> From: =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28Intel=29?= Message-ID: <6c9acb90-8e91-d8af-7abd-e762d9a901aa@shipmail.org> Date: Wed, 24 Mar 2021 14:35:38 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210324124127.GY2356281@nvidia.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Stat-Signature: 6oz7dycf3iffw1c8f6qytbdwe8bmgjhj X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 489F4E0011ED Received-SPF: none (shipmail.org>: No applicable sender policy available) receiver=imf05; identity=mailfrom; envelope-from=""; helo=pio-pvt-msa1.bahnhof.se; client-ip=79.136.2.40 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616592945-876332 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 3/24/21 1:41 PM, Jason Gunthorpe wrote: > On Wed, Mar 24, 2021 at 01:35:17PM +0100, Thomas Hellstr=C3=B6m (Intel)= wrote: >> On 3/24/21 1:24 PM, Jason Gunthorpe wrote: >>> On Wed, Mar 24, 2021 at 10:56:43AM +0100, Daniel Vetter wrote: >>>> On Tue, Mar 23, 2021 at 06:06:53PM +0100, Thomas Hellstr=C3=B6m (Int= el) wrote: >>>>> On 3/23/21 5:37 PM, Jason Gunthorpe wrote: >>>>>> On Tue, Mar 23, 2021 at 05:34:51PM +0100, Thomas Hellstr=C3=B6m (I= ntel) wrote: >>>>>> >>>>>>>>> @@ -210,6 +211,20 @@ static vm_fault_t ttm_bo_vm_insert_huge(st= ruct vm_fault *vmf, >>>>>>>>> if ((pfn & (fault_page_size - 1)) !=3D 0) >>>>>>>>> goto out_fallback; >>>>>>>>> + /* >>>>>>>>> + * Huge entries must be special, that is marking them as devm= ap >>>>>>>>> + * with no backing device map range. If there is a backing >>>>>>>>> + * range, Don't insert a huge entry. >>>>>>>>> + * If this check turns out to be too much of a performance hi= t, >>>>>>>>> + * we can instead have drivers indicate whether they may have >>>>>>>>> + * backing device map ranges and if not, skip this lookup. >>>>>>>>> + */ >>>>>>>> I think we can do this statically: >>>>>>>> - if it's system memory we know there's no devmap for it, and we= do the >>>>>>>> trick to block gup_fast >>>>>>> Yes, that should work. >>>>>>>> - if it's iomem, we know gup_fast wont work anyway if don't set = PFN_DEV, >>>>>>>> so might as well not do that >>>>>>> I think gup_fast will unfortunately mistake a huge iomem page for= an >>>>>>> ordinary page and try to access a non-existant struct page for it= , unless we >>>>>>> do the devmap trick. >>>>>>> >>>>>>> And the lookup would then be for the rare case where a driver wou= ld have >>>>>>> already registered a dev_pagemap for an iomem area which may also= be mapped >>>>>>> through TTM (like the patch from Felix a couple of weeks ago). If= a driver >>>>>>> can promise not to do that, then we can safely remove the lookup. >>>>>> Isn't the devmap PTE flag arch optional? Does this fall back to no= t >>>>>> using huge pages on arches that don't support it? >>>>> Good point. No, currently it's only conditioned on transhuge page s= upport. >>>>> Need to condition it on also devmap support. >>>>> >>>>>> Also, I feel like this code to install "pte_special" huge pages do= es >>>>>> not belong in the drm subsystem.. >>>>> I could add helpers in huge_memory.c: >>>>> >>>>> vmf_insert_pfn_pmd_prot_special() and >>>>> vmf_insert_pfn_pud_prot_special() >>>> The somewhat annoying thing is that we'd need an error code so we fa= ll >>>> back to pte fault handling. That's at least my understanding of how >>>> pud/pmd fault handling works. Not sure how awkward that is going to = be >>>> with the overall fault handling flow. >>>> >>>> But aside from that I think this makes tons of sense. >>> Why should the driver be so specific? >>> >>> vmf_insert_pfn_range_XXX() >>> >>> And it will figure out the optimal way to build the page tables. >>> >>> Driver should provide the largest physically contiguous range it can >> I figure that would probably work, but since the huge_fault() interfac= e is >> already providing the size of the fault based on how the pagetable is >> currently populated I figure that would have to move a lot of that log= ic >> into that helper... > But we don't really care about the size of the fault when we stuff the > pfns. > > The device might use it when handling the fault, but once the fault is > handled the device knows what the contiguous pfn range is that it has > available to stuff into the page tables, it just tells the vmf_insert > what it was able to create, and it creates the necessary page table > structure. > > The size of the hole in the page table is really only advisory, the > device may not want to make a 2M or 1G page entry and may prefer to > only create 4k. > > In an ideal world the creation/destruction of page table levels would > by dynamic at this point, like THP. Hmm, but I'm not sure what problem we're trying to solve by changing the=20 interface in this way? Currently if the core vm requests a huge pud, we give it one, and if we=20 can't or don't want to (because of dirty-tracking, for example, which is=20 always done on 4K page-level) we just return VM_FAULT_FALLBACK, and the=20 fault is retried at a lower level. Also, determining whether we have a=20 contigous range is not free, so we=C2=A0 don't want to do that unnecessar= ily. /Thomas