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 35261C4332F for ; Fri, 1 Oct 2021 22:22:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 197EC61A8E for ; Fri, 1 Oct 2021 22:22:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240743AbhJAWYg (ORCPT ); Fri, 1 Oct 2021 18:24:36 -0400 Received: from ale.deltatee.com ([204.191.154.188]:36804 "EHLO ale.deltatee.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230171AbhJAWYf (ORCPT ); Fri, 1 Oct 2021 18:24:35 -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=ent50A2FumpC9aDXR5Qm0QH4qyHyD1xD/705x8AcEeY=; b=s/z7F8PLrEmDP31iQENlxVoSTT 4H9l1aoVN8hef71V44S4SxGoamROPJy+o+t5Inv+9wUfb0JlZBmRG9IP5Q5pkDIiMhbUs2EWCzOQD ij7Nano0aq6SDFRlJb1aqMkZUYSsEE5MPUok4lkBY/76ZdxutXBGBkZlvnvzvMarVcbrwZUiJHy8x hz9y+5idzSe5S/rECi/Cde14mXwHV6WqVx3bDj4NrGxyLns/zKclaYP5wBoByv2HQy7cZ/+ZlvwJ2 BmRbZwnBFl3l22PHfA1sw7r1YVy4zXYebzwz0wDI8zcvW670GNrgnJ5WAVHpcxCuzGJqAI4LD3eTx ng/1j+9w==; 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 1mWQvK-0000Wo-1S; Fri, 01 Oct 2021 16:22:35 -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: <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> <95ada0ac-08cc-5b77-8675-b955b1b6d488@deltatee.com> <20211001221405.GR3544071@ziepe.ca> From: Logan Gunthorpe Message-ID: <8871549c-63b5-d062-87ea-9036605984d5@deltatee.com> Date: Fri, 1 Oct 2021 16:22:28 -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: <20211001221405.GR3544071@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 4:14 p.m., Jason Gunthorpe wrote: > On Fri, Oct 01, 2021 at 02:13:14PM -0600, Logan Gunthorpe wrote: >> >> >> 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. > > It would close this issue, however synchronize_rcu() is very slow > (think > 1second) in some cases and thus cannot be inserted here. It shouldn't be *that* slow, at least not the vast majority of the time... it seems a bit unreasonable that a CPU wouldn't schedule for more than a second. But these aren't fast paths and synchronize_rcu() already gets called in the unbind path for p2pdma a of couple times. I'm sure it would also be fine to slow down the vma_close() path as well. > I'm also not completely sure that rcu is the only case, I don't know > how every arch handles its gather structure.. I have a feeling the > general intention was for this to be asynchronous Yeah, this is not clear to me either. > My preferences are to either remove devmap from gup_fast, or fix it to > not use special pages - the latter being obviously better. Yeah, I rather expect DAX users want the optimization provided by gup_fast. I don't think P2PDMA users would be happy about being stuck with slow gup either. Loga