From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f70.google.com (mail-ot1-f70.google.com [209.85.210.70]) by kanga.kvack.org (Postfix) with ESMTP id 06A156B53F1 for ; Thu, 29 Nov 2018 13:51:20 -0500 (EST) Received: by mail-ot1-f70.google.com with SMTP id v34so1432619ote.7 for ; Thu, 29 Nov 2018 10:51:20 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id q13sor1274665otm.98.2018.11.29.10.51.18 for (Google Transport Security); Thu, 29 Nov 2018 10:51:18 -0800 (PST) MIME-Version: 1.0 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> <43778343-6d43-eb43-0de0-3db6828902d0@deltatee.com> In-Reply-To: <43778343-6d43-eb43-0de0-3db6828902d0@deltatee.com> From: Dan Williams Date: Thu, 29 Nov 2018 10:51:06 -0800 Message-ID: Subject: Re: [PATCH v8 3/7] mm, devm_memremap_pages: Fix shutdown handling Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Logan Gunthorpe 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 Thu, Nov 29, 2018 at 9:51 AM Logan Gunthorpe wrote: > > > > 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. True, it is... > Though, if we move the > arch_remove_pages() into a different context, it *may* help with the > problem below... Glad to see my over-engineered proposal in this case might be good for something... > > >> 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. > Got it, let me see how bad moving arch_remove_memory() turns out, sounds like a decent approach to coordinate multiple users of a single ref.