From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io1-f72.google.com (mail-io1-f72.google.com [209.85.166.72]) by kanga.kvack.org (Postfix) with ESMTP id 66C4A6B53CE for ; Thu, 29 Nov 2018 12:50:55 -0500 (EST) Received: by mail-io1-f72.google.com with SMTP id d63so2714360iog.4 for ; Thu, 29 Nov 2018 09:50:55 -0800 (PST) Received: from ale.deltatee.com (ale.deltatee.com. [207.54.116.67]) by mx.google.com with ESMTPS id b17si1564697ioc.42.2018.11.29.09.50.53 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 29 Nov 2018 09:50:54 -0800 (PST) References: <154275556908.76910.8966087090637564219.stgit@dwillia2-desk3.amr.corp.intel.com> <154275558526.76910.7535251937849268605.stgit@dwillia2-desk3.amr.corp.intel.com> <6875ca04-a36a-89ae-825b-f629ab011d47@deltatee.com> <14d6413c-b002-c152-5016-7ed659c08c24@deltatee.com> From: Logan Gunthorpe Message-ID: <43778343-6d43-eb43-0de0-3db6828902d0@deltatee.com> Date: Thu, 29 Nov 2018 10:50:46 -0700 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-CA Content-Transfer-Encoding: 7bit Subject: Re: [PATCH v8 3/7] mm, devm_memremap_pages: Fix shutdown handling Sender: owner-linux-mm@kvack.org List-ID: To: Dan Williams Cc: Andrew Morton , stable , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Christoph Hellwig , Linus Torvalds , Linux MM , Linux Kernel Mailing List , Maling list - DRI developers , Bjorn Helgaas , Stephen Bates On 2018-11-29 10:30 a.m., Dan Williams wrote: > Oh! Yes, nice find. We need to wait for the percpu-ref to be dead and > all outstanding references dropped before we can proceed to > arch_remove_memory(), and I think this problem has been there since > day one because the final exit was always after devm_memremap_pages() > release which means arch_remove_memory() was always racing any final > put_page(). I'll take a look, it seems the arch_remove_pages() call > needs to be moved out-of-line to its own context and wait for the > final exit of the percpu-ref. Ok, well I thought moving the wait_for_completion() into the kill() call was a pretty good solution to this. Though, if we move the arch_remove_pages() into a different context, it *may* help with the problem below... >> Though, now that I look at it, the current change in question will be >> wrong if there are two devm_memremap_pages_release()s to call. Both need >> to drop their references before we can wait_for_completion() ;(. I guess >> I need multiple percpu_refs or more complex changes to >> devm_memremap_pages_release(). > > Can you just have a normal device-level kref for this case? On final > device-level kref_put then kill the percpu_ref? I guess the problem is > devm semantics where p2pdma only gets one callback on a driver > ->remove() event. I'm not sure how to support multiple references of > the same pages without creating a non-devm version of > devm_memremap_pages(). I'm not opposed to that, but afaiu I don't > think p2pdma is compatible with devm as long as it supports N>1:1 > mappings of the same range. Hmm, no I think you misunderstood what I said. I'm saying I need to have exactly one percpu_ref per call to devm_memremap_pages() and this is doable, just slightly annoying. Right now I have one percpu_ref for multiple calls to devm_memremap_pages() which doesn't work with the above fix because there will always be a wait_for_completion() before the last references are dropped in this way: 1) First devm_memremap_pages_release() is called which drops it's reference and waits_for_completion(). 2) The second devm_memremap_pages_release() needs to be called to drop it's reference, but can't seeing the first is waiting, and therefore the percpu_ref never goes to zero and the wait_for_completion() never returns. Logan