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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 251FBC5ACAE for ; Wed, 11 Sep 2019 15:08:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C2EB62084F for ; Wed, 11 Sep 2019 15:08:47 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=shipmail.org header.i=@shipmail.org header.b="UnStjuBW" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728330AbfIKPIr (ORCPT ); Wed, 11 Sep 2019 11:08:47 -0400 Received: from ste-pvt-msa2.bahnhof.se ([213.80.101.71]:58151 "EHLO ste-pvt-msa2.bahnhof.se" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726510AbfIKPIq (ORCPT ); Wed, 11 Sep 2019 11:08:46 -0400 Received: from localhost (localhost [127.0.0.1]) by ste-pvt-msa2.bahnhof.se (Postfix) with ESMTP id 4EFA640E8D; Wed, 11 Sep 2019 17:08:39 +0200 (CEST) Authentication-Results: ste-pvt-msa2.bahnhof.se; dkim=pass (1024-bit key; unprotected) header.d=shipmail.org header.i=@shipmail.org header.b=UnStjuBW; dkim-atps=neutral X-Virus-Scanned: Debian amavisd-new at bahnhof.se Authentication-Results: ste-ftg-msa2.bahnhof.se (amavisd-new); dkim=pass (1024-bit key) header.d=shipmail.org Received: from ste-pvt-msa2.bahnhof.se ([127.0.0.1]) by localhost (ste-ftg-msa2.bahnhof.se [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id K4cCLt3qtFDL; Wed, 11 Sep 2019 17:08:38 +0200 (CEST) Received: from mail1.shipmail.org (h-205-35.A357.priv.bahnhof.se [155.4.205.35]) (Authenticated sender: mb878879) by ste-pvt-msa2.bahnhof.se (Postfix) with ESMTPA id E33AE40E84; Wed, 11 Sep 2019 17:08:37 +0200 (CEST) Received: from localhost.localdomain (h-205-35.A357.priv.bahnhof.se [155.4.205.35]) by mail1.shipmail.org (Postfix) with ESMTPSA id 546AB3601AA; Wed, 11 Sep 2019 17:08:37 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=shipmail.org; s=mail; t=1568214517; bh=iXS3GPpxTpsS/0BsV1h1Z1UOn1deQj53Xl4MDL0r1Es=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=UnStjuBWdFqAiDUOxNAooUYh9qTARPLWsZrAun876cQiysskITkouXSkMiV9hNo9H yUQ1AuE1dL883RxQjgOEFYXego1hdN5ij+QaDi4kdMiVubKj0d7SHtsa6oTePemL5g iollE2oXywNd3+UsQkcPONkYX234t7/uKfgSe6Qo= Subject: Re: TTM huge page-faults WAS: Re: [RFC PATCH 1/2] x86: Don't let pgprot_modify() change the page encryption bit To: "Koenig, Christian" Cc: "linux-kernel@vger.kernel.org" , "pv-drivers@vmware.com" , Thomas Hellstrom , "dri-devel@lists.freedesktop.org" References: <20190905103541.4161-1-thomas_os@shipmail.org> <20190905103541.4161-2-thomas_os@shipmail.org> <608bbec6-448e-f9d5-b29a-1984225eb078@intel.com> <20190905152438.GA18286@infradead.org> <10185AAF-BFB8-4193-A20B-B97794FB7E2F@amacapital.net> <92171412-eed7-40e9-2554-adb358e65767@shipmail.org> From: =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28VMware=29?= Organization: VMware Inc. Message-ID: <46a21ca5-a395-e644-9eed-77ea26fc871e@shipmail.org> Date: Wed, 11 Sep 2019 17:08:36 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/11/19 4:06 PM, Koenig, Christian wrote: > Am 11.09.19 um 12:10 schrieb Thomas Hellström (VMware): > [SNIP] >>>> The problem seen in TTM is that we want to be able to change the >>>> vm_page_prot from the fault handler, but it's problematic since we >>>> have the mmap_sem typically only in read mode. Hence the fake vma >>>> hack. From what I can tell it's reasonably well-behaved, since >>>> pte_modify() skips the bits TTM updates, so mprotect() and mremap() >>>> works OK. I think split_huge_pmd may run into trouble, but we don't >>>> support it (yet) with TTM. >>> Ah! I actually ran into this while implementing huge page support for >>> TTM and never figured out why that doesn't work. Dropped CPU huge page >>> support because of this. >> By incident, I got slightly sidetracked the other day and started >> looking at this as well. Got to the point where I figured out all the >> hairy alignment issues and actually got huge_fault() calls, but never >> implemented the handler. I think that's definitely something worth >> having. Not sure it will work for IO memory, though, (split_huge_pmd >> will just skip non-page-backed memory) but if we only support >> VM_SHARED (non COW) vmas there's no reason to split the huge pmds >> anyway. Definitely something we should have IMO. > Well our primary use case would be IO memory, cause system memory is > only optionally allocate as huge page but we nearly always allocate VRAM > in chunks of at least 2MB because we otherwise get a huge performance > penalty. But that system memory option is on by default, right? In any case, a request for a huge_fault would probably need to check that there is actually an underlying huge_page and otherwise fallback to ordinary faults. Another requirement would be for IO memory allocations to be PMD_PAGE_SIZE aligned in the mappable aperture, to avoid fallbacks to ordinary faults. Probably increasing fragmentation somewhat. (Seems like pmd entries can only point to PMD_PAGE_SIZE aligned physical addresses) Would that work for you? >>>> We could probably get away with a WRITE_ONCE() update of the >>>> vm_page_prot before taking the page table lock since >>>> >>>> a) We're locking out all other writers. >>>> b) We can't race with another fault to the same vma since we hold an >>>> address space lock ("buffer object reservation") >>>> c) When we need to update there are no valid page table entries in the >>>> vma, since it only happens directly after mmap(), or after an >>>> unmap_mapping_range() with the same address space lock. When another >>>> reader (for example split_huge_pmd()) sees a valid page table entry, >>>> it also sees the new page protection and things are fine. >>> Yeah, that's exactly why I always wondered why we need this hack with >>> the vma copy on the stack. >>> >>>> But that would really be a special case. To solve this properly we'd >>>> probably need an additional lock to protect the vm_flags and >>>> vm_page_prot, taken after mmap_sem and i_mmap_lock. >>> Well we already have a special lock for this: The reservation object. So >>> memory barriers etc should be in place and I also think we can just >>> update the vm_page_prot on the fly. >> I agree. This is needed for huge pages. We should make this change, >> and perhaps add the justification above as a comment. > Alternatively we could introduce a new VM_* flag telling users of > vm_page_prot to just let the pages table entries be filled by faults again An interesting idea, although we'd lose things like dirty-tracking bits. /Thomas > > Christian.