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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EC9E6C433FE for ; Fri, 1 Oct 2021 20:13:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CD67E61AF7 for ; Fri, 1 Oct 2021 20:13:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229896AbhJAUPY (ORCPT ); Fri, 1 Oct 2021 16:15:24 -0400 Received: from ale.deltatee.com ([204.191.154.188]:35274 "EHLO ale.deltatee.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229879AbhJAUPX (ORCPT ); Fri, 1 Oct 2021 16:15:23 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=deltatee.com; s=20200525; h=Subject:In-Reply-To:MIME-Version:Date: Message-ID:From:References:Cc:To:content-disposition; bh=MT0f0inatgvWHt9y6q41qCoiZgUB0hemG4eYA9iG0ZI=; b=gqh8dihCcDiD7/l4vhuUnLDPle 9EFzkzLPUy1dTkJIIS1JcbckiRfeXzJ3oPIWHHWu13MU5vZZTHs9K3VGis/f5lP/HAvCEkb2bDCoN TVp36+euwLuf/+pQqIkLxjeOadBmDXsgBbgQIXRtYh3SCtOVuof43ew53y5WYQK1MoKHHMWFWrxpj YV8lkp6q6VCo8sHUFpLphXcyAeg2LlwFDBTBjy2/jUe8+CL2T7gCQhwH9jY3nmRmA9+owtn82wpNH GNBBBIIzV1eUB0QsANCVQCY6M7rzdY6M85FAsLTdYcDm8qwoj53La4rnmlpfbJDQo7Qm8m25f65pS GXXa4xSA==; Received: from s0106a84e3fe8c3f3.cg.shawcable.net ([24.64.144.200] helo=[192.168.0.10]) by ale.deltatee.com with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1mWOuH-000696-1t; Fri, 01 Oct 2021 14:13:22 -0600 To: Jason Gunthorpe Cc: Alistair Popple , Felix Kuehling , Christoph Hellwig , Dan Williams , linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-pci@vger.kernel.org, linux-mm@kvack.org, iommu@lists.linux-foundation.org, Stephen Bates , =?UTF-8?Q?Christian_K=c3=b6nig?= , John Hubbard , Don Dutile , Matthew Wilcox , Daniel Vetter , Jakowski Andrzej , Minturn Dave B , Jason Ekstrand , Dave Hansen , Xiong Jianxin , Bjorn Helgaas , Ira Weiny , Robin Murphy , Martin Oliveira , Chaitanya Kulkarni References: <20210916234100.122368-20-logang@deltatee.com> <20210928195518.GV3544071@ziepe.ca> <8d386273-c721-c919-9749-fc0a7dc1ed8b@deltatee.com> <20210929230543.GB3544071@ziepe.ca> <32ce26d7-86e9-f8d5-f0cf-40497946efe9@deltatee.com> <20210929233540.GF3544071@ziepe.ca> <20210930003652.GH3544071@ziepe.ca> <20211001134856.GN3544071@ziepe.ca> <4fdd337b-fa35-a909-5eee-823bfd1e9dc4@deltatee.com> <20211001174511.GQ3544071@ziepe.ca> From: Logan Gunthorpe Message-ID: <95ada0ac-08cc-5b77-8675-b955b1b6d488@deltatee.com> Date: Fri, 1 Oct 2021 14:13:14 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 MIME-Version: 1.0 In-Reply-To: <20211001174511.GQ3544071@ziepe.ca> Content-Type: text/plain; charset=utf-8 Content-Language: en-CA Content-Transfer-Encoding: 7bit X-SA-Exim-Connect-IP: 24.64.144.200 X-SA-Exim-Rcpt-To: ckulkarnilinux@gmail.com, martin.oliveira@eideticom.com, robin.murphy@arm.com, ira.weiny@intel.com, helgaas@kernel.org, jianxin.xiong@intel.com, dave.hansen@linux.intel.com, jason@jlekstrand.net, dave.b.minturn@intel.com, andrzej.jakowski@intel.com, daniel.vetter@ffwll.ch, willy@infradead.org, ddutile@redhat.com, jhubbard@nvidia.com, christian.koenig@amd.com, sbates@raithlin.com, iommu@lists.linux-foundation.org, linux-mm@kvack.org, linux-pci@vger.kernel.org, linux-block@vger.kernel.org, linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org, dan.j.williams@intel.com, hch@lst.de, Felix.Kuehling@amd.com, apopple@nvidia.com, jgg@ziepe.ca X-SA-Exim-Mail-From: logang@deltatee.com Subject: Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem() X-SA-Exim-Version: 4.2.1 (built Wed, 08 May 2019 21:11:16 +0000) X-SA-Exim-Scanned: Yes (on ale.deltatee.com) Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 2021-10-01 11:45 a.m., Jason Gunthorpe wrote: >> Before the invalidation, an active flag is cleared to ensure no new >> mappings can be created while the unmap is proceeding. >> unmap_mapping_range() should sequence itself with the TLB flush and > > AFIAK unmap_mapping_range() kicks off the TLB flush and then > returns. It doesn't always wait for the flush to fully finish. Ie some > cases use RCU to lock the page table against GUP fast and so the > put_page() doesn't happen until the call_rcu completes - after a grace > period. The unmap_mapping_range() does not wait for grace periods. Admittedly, the tlb flush code isn't the easiest code to understand. But, yes it seems at least on some arches the pages are freed by call_rcu(). But can't this be fixed easily by adding a synchronize_rcu() call after calling unmap_mapping_range()? Certainly after a synchronize_rcu(), the TLB has been flushed and it is safe to free those pages. >> P2PDMA follows this pattern, except pages are not mapped linearly and >> are returned to the genalloc when their refcount falls to 1. This only >> happens after a VMA is closed which should imply the PTEs have already >> been unlinked from the pages. > > And here is the problem, since the genalloc is being used we now care > that a page should not continue to be accessed by userspace after it > has be placed back into the genalloc. I suppose fsdax has the same > basic issue too. Ok, similar question. Then if we call synchronize_rcu() in vm_close(), before the put_page() calls which return the pages to the genalloc, would that not guarantee the TLBs have been appropriately flushed? >> Not to say that all this couldn't use a big conceptual cleanup. A >> similar question exists with the single find_special_page() user >> (xen/gntdev) and it's definitely not clear what the differences are >> between the find_special_page() and vmf_insert_mixed() techniques and >> when one should be used over the other. Or could they both be merged to >> use the same technique? > > Oh that gntdev stuff is just nonsense. IIRC is trying to delegate > control over a PTE entry itself to the hypervisor. > > /* > * gntdev takes the address of the PTE in find_grant_ptes() and > * passes it to the hypervisor in gntdev_map_grant_pages(). The > * purpose of the notifier is to prevent the hypervisor pointer > * to the PTE from going stale. > * > * Since this vma's mappings can't be touched without the > * mmap_lock, and we are holding it now, there is no need for > * the notifier_range locking pattern. > > I vaugely recall it stuffs in a normal page then has the hypervisor > overwrite the PTE. When it comes time to free the PTE it recovers the > normal page via the 'find_special_page' hack and frees it. Somehow the > hypervisor is also using the normal page for something. > > It is all very strange and one shouldn't think about it :| Found this from an old commit message which seems to be a better explanation, though I still don't fully understand it: In a Xen PV guest, the PTEs contain MFNs so get_user_pages() (for example) must do an MFN to PFN (M2P) lookup before it can get the page. For foreign pages (those owned by another guest) the M2P lookup returns the PFN as seen by the foreign guest (which would be completely the wrong page for the local guest). This cannot be fixed up improving the M2P lookup since one MFN may be mapped onto two or more pages so getting the right page is impossible given just the MFN. Yes, all very strange. Logan