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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id BCE20C54731 for ; Tue, 27 Aug 2024 16:12:23 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1siyn5-0007eJ-U5; Tue, 27 Aug 2024 12:11:31 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1siyn4-0007cZ-1D for qemu-devel@nongnu.org; Tue, 27 Aug 2024 12:11:30 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1siyn0-0007YH-HF for qemu-devel@nongnu.org; Tue, 27 Aug 2024 12:11:29 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1724775082; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Emi7GzldYpNp3zL7GMCh31+uwJG1ZeK0oNtY8PDHCAM=; b=TOiO9eXrnTW2Qf64e3+nYIGxD2oFeOxVIM5b/A+dVwpaXWp+Yjl2+yZpZhKO0JASsvb8lq ZfjjITiTngIvaq0W14kSXz0jhclI+Gpyo2m+Bv3UZREYwbZRjYxmItK1/Is5GSBfmRh2TW J42xrBvpdLJ5LfKkOS88SSJVjVFOxHs= Received: from mail-qt1-f197.google.com (mail-qt1-f197.google.com [209.85.160.197]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-464-7oAtVqOGOICDFb05v_ahtg-1; Tue, 27 Aug 2024 12:11:18 -0400 X-MC-Unique: 7oAtVqOGOICDFb05v_ahtg-1 Received: by mail-qt1-f197.google.com with SMTP id d75a77b69052e-44ffce7ed6aso82213691cf.3 for ; Tue, 27 Aug 2024 09:11:18 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1724775078; x=1725379878; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=Emi7GzldYpNp3zL7GMCh31+uwJG1ZeK0oNtY8PDHCAM=; b=ticidv1MTPNqcVcrRAwcCIn/VcY4TO709VGYSk6QmETK4OWv09yuforKKPs0KtkJdI xlvs/0C45NRuOzB148K1I9RNJudeZXkHKu2zAYp3KnZsExMoUedEA4TAvsFO/u/aD6uV sTDihKHDzhtpLdGmmkP5NRqQ4xnQGHNlkMOTDNor4GRHNIESW+wFaeegCjdtYGmLEH6y gXLnppuBOclvLJX0HbB7f/5I159oj+H2TZJ3ANBhpKPmT7AT/LxEOo0H7zKILE8t/wlo 5zLGcchUjFcA2ncAz1xpbjxGn0NfDQ+MuzlaZQX/pPxwMxWl1NXkXhi0MA3xQfUbuP/q mZng== X-Forwarded-Encrypted: i=1; AJvYcCUQRR+bHNp7Czz6mht4Wq7Y1jp4WJHf6a12xTxqJGUes607IUg2mG1LKbG9KDyj6x4bldxv1k/OqGZa@nongnu.org X-Gm-Message-State: AOJu0YxU6REuzN9Gck3yAMKlIXIgnwgpaPK+Oz72zxBdr4kuR1IS/6i9 9fpuk2zMKuQaHesP0lA2tVcsdHpMsLyE0vl9IGwuFbhxmxNUr2H2sZUi7HpDUAAW9JKF+FTKza0 ke1w5b2rHpfwxhpcdCLMQNYL9ufd0xv90d4lepp4gNAeXX7vRe5fy X-Received: by 2002:a05:622a:4d95:b0:447:f469:b58d with SMTP id d75a77b69052e-455097bc89cmr155564811cf.42.1724775077689; Tue, 27 Aug 2024 09:11:17 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEZC8dHsoGQmPp4jXL6gcFd4xcw8Ev9lhouj12fkhhPHLtba3PIeFskWezv42IDgJRabMH/5A== X-Received: by 2002:a05:622a:4d95:b0:447:f469:b58d with SMTP id d75a77b69052e-455097bc89cmr155564371cf.42.1724775077259; Tue, 27 Aug 2024 09:11:17 -0700 (PDT) Received: from x1n (pool-99-254-121-117.cpe.net.cable.rogers.com. [99.254.121.117]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-454fe0f436dsm54534591cf.49.2024.08.27.09.11.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Aug 2024 09:11:16 -0700 (PDT) Date: Tue, 27 Aug 2024 12:11:13 -0400 From: Peter Xu To: Akihiko Odaki Cc: Peter Maydell , Eduardo Habkost , Marcel Apfelbaum , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Yanan Wang , John Snow , BALATON Zoltan , Jiaxun Yang , Nicholas Piggin , Daniel Henrique Barboza , David Gibson , Harsh Prateek Bora , Alexey Kardashevskiy , "Michael S. Tsirkin" , Alex =?utf-8?Q?Benn=C3=A9e?= , Fabiano Rosas , Paolo Bonzini , David Hildenbrand , Thomas Huth , Laurent Vivier , qemu-devel@nongnu.org, qemu-block@nongnu.org, qemu-ppc@nongnu.org Subject: Re: [PATCH v4 6/7] memory: Do not create circular reference with subregion Message-ID: References: <20240823-san-v4-0-a24c6dfa4ceb@daynix.com> <20240823-san-v4-6-a24c6dfa4ceb@daynix.com> <161cb8ff-1479-4fc4-8803-d665e757007a@daynix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <161cb8ff-1479-4fc4-8803-d665e757007a@daynix.com> Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Aug 27, 2024 at 01:14:51PM +0900, Akihiko Odaki wrote: > On 2024/08/27 4:42, Peter Xu wrote: > > On Mon, Aug 26, 2024 at 06:10:25PM +0100, Peter Maydell wrote: > > > On Mon, 26 Aug 2024 at 16:22, Peter Xu wrote: > > > > > > > > On Fri, Aug 23, 2024 at 03:13:11PM +0900, Akihiko Odaki wrote: > > > > > memory_region_update_container_subregions() used to call > > > > > memory_region_ref(), which creates a reference to the owner of the > > > > > subregion, on behalf of the owner of the container. This results in a > > > > > circular reference if the subregion and container have the same owner. > > > > > > > > > > memory_region_ref() creates a reference to the owner instead of the > > > > > memory region to match the lifetime of the owner and memory region. We > > > > > do not need such a hack if the subregion and container have the same > > > > > owner because the owner will be alive as long as the container is. > > > > > Therefore, create a reference to the subregion itself instead ot its > > > > > owner in such a case; the reference to the subregion is still necessary > > > > > to ensure that the subregion gets finalized after the container. > > > > > > > > > > Signed-off-by: Akihiko Odaki > > > > > --- > > > > > system/memory.c | 8 ++++++-- > > > > > 1 file changed, 6 insertions(+), 2 deletions(-) > > > > > > > > > > diff --git a/system/memory.c b/system/memory.c > > > > > index 5e6eb459d5de..e4d3e9d1f427 100644 > > > > > --- a/system/memory.c > > > > > +++ b/system/memory.c > > > > > @@ -2612,7 +2612,9 @@ static void memory_region_update_container_subregions(MemoryRegion *subregion) > > > > > > > > > > memory_region_transaction_begin(); > > > > > > > > > > - memory_region_ref(subregion); > > > > > + object_ref(mr->owner == subregion->owner ? > > > > > + OBJECT(subregion) : subregion->owner); > > > > > > > > The only place that mr->refcount is used so far is the owner with the > > > > object property attached to the mr, am I right (ignoring name-less MRs)? > > > > > > > > I worry this will further complicate refcounting, now we're actively using > > > > two refcounts for MRs.. > > The actor of object_ref() is the owner of the memory region also in this > case. We are calling object_ref() on behalf of mr->owner so we use > mr->refcount iff mr->owner == subregion->owner. In this sense there is only > one user of mr->refcount even after this change. Yes it's still one user, but it's not that straightforward to see, also it's still an extension to how we use mr->refcount right now. Currently it's about "true / false" just to describe, now it's a real counter. I wished that counter doesn't even exist if we'd like to stick with device / owner's counter. Adding this can definitely also make further effort harder if we want to remove mr->refcount. > > > > > > > > > Continue discussion there: > > > > > > > > https://lore.kernel.org/r/067b17a4-cdfc-4f7e-b7e4-28c38e1c10f0@daynix.com > > > > > > > > What I don't see is how mr->subregions differs from mr->container, so we > > > > allow subregions to be attached but not the container when finalize() > > > > (which is, afaict, the other way round). > > > > > > > > It seems easier to me that we allow both container and subregions to exist > > > > as long as within the owner itself, rather than start heavier use of > > > > mr->refcount. > > > > > > I don't think just "same owner" necessarily will be workable -- > > > you can have a setup like: > > > * device A has a container C_A > > > * device A has a child-device B > > > * device B has a memory region R_B > > > * device A's realize method puts R_B into C_A > > > > > > R_B's owner is B, and the container's owner is A, > > > but we still want to be able to get rid of A (in the process > > > getting rid of B because it gets unparented and unreffed, > > > and R_B and C_A also). > > > > For cross-device references, should we rely on an explicit call to > > memory_region_del_subregion(), so as to detach the link between C_A and > > R_B? > > Yes, I agree. > > > > > My understanding so far: logically when MR finalize() it should guarantee > > both (1) mr->container==NULL, and (2) mr->subregions empty. That's before > > commit 2e2b8eb70fdb7dfb and could be the ideal world (though at the very > > beginning we don't assert on ->container==NULL yet). It requires all > > device emulations to do proper unrealize() to unlink all the MRs. > > > > However what I'm guessing is QEMU probably used to have lots of devices > > that are not following the rules and leaking these links. Hence we have > > had 2e2b8eb70fdb7dfb, allowing that to happen as long as it's safe, and > > it's justified by comment in 2e2b8eb70fdb7dfb on why it's safe. > > > > What I was thinking is this comment seems to apply too to mr->container, so > > that it should be safe too to unlink ->container the same way as its own > > subregions. > > > IIUC that means for device-internal MR links we should be fine leaving > > whatever link between MRs owned by such device; the device->refcount > > guarantees none of them will be visible in any AS. But then we need to > > always properly unlink the MRs when the link is across >1 device owners, > > otherwise it's prone to leak. > > There is one principle we must satisfy in general: keep a reference to a > memory region if it is visible to the guest. > > It is safe to call memory_region_del_subregion() and to trigger the > finalization of subregions when the container is not referenced because they > are no longer visible. This is not true for the other way around; even when > subregions are not referenced by anyone else, they are still visible to the > guest as long as the container is visible to the guest. It is not safe to > unref and finalize them in such a case. > > A memory region and its owner will leak if a memory region kept visible for > a too long period whether the chain of reference contains a > container/subregion relationship or not. Could you elaborate why it's still visible to the guest if owner->refcount==0 && mr->container!=NULL? Firstly, mr->container != NULL means the MR has an user indeed. It's the matter of who's using it. If that came from outside this device, it should require memory_region_ref(mr) before hand when adding the subregion, and that will hold one reference on the owner->refcount. Here owner->refcount==0 means there's no such reference, so it seems to me it's guaranteed to not be visible to anything outside of this device / owner. Then from that POV it's safe to unlink when the owner is finalizing just like what we do with mr->subregions, no? -- Peter Xu