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Fri, 07 Feb 2020 10:17:52 -0800 (PST) X-Google-Smtp-Source: APXvYqzRox8Bo7fyFYmNOjYNh/V3B0M3O5rtHPF6if8Y080MuT3a3U4ayt/3NWDSUCkErG4IzY/vdw== X-Received: by 2002:a05:6214:1253:: with SMTP id q19mr106704qvv.75.1581099471868; Fri, 07 Feb 2020 10:17:51 -0800 (PST) Received: from xz-x1 ([2607:9880:19c8:32::2]) by smtp.gmail.com with ESMTPSA id b24sm1725899qto.71.2020.02.07.10.17.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 07 Feb 2020 10:17:51 -0800 (PST) Date: Fri, 7 Feb 2020 13:17:48 -0500 From: Peter Xu To: Sean Christopherson Subject: Re: [PATCH v5 12/19] KVM: Move memslot deletion to helper function Message-ID: <20200207181748.GC720553@xz-x1> References: <20200121223157.15263-1-sean.j.christopherson@intel.com> <20200121223157.15263-13-sean.j.christopherson@intel.com> <20200206161415.GA695333@xz-x1> <20200206162818.GD13067@linux.intel.com> <20200206165116.GE695333@xz-x1> <20200207175912.GG2401@linux.intel.com> MIME-Version: 1.0 In-Reply-To: <20200207175912.GG2401@linux.intel.com> X-MC-Unique: b3gXl4w2P6yE44wHINc9gw-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200207_101756_807093_F0E4C186 X-CRM114-Status: GOOD ( 26.89 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Wanpeng Li , kvm@vger.kernel.org, David Hildenbrand , linux-mips@vger.kernel.org, Paul Mackerras , kvmarm@lists.cs.columbia.edu, Janosch Frank , Marc Zyngier , Joerg Roedel , Christian Borntraeger , Julien Thierry , Suzuki K Poulose , kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Jim Mattson , Cornelia Huck , Christoffer Dall , linux-kernel@vger.kernel.org, James Morse , Paolo Bonzini , Vitaly Kuznetsov , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Feb 07, 2020 at 09:59:12AM -0800, Sean Christopherson wrote: > On Thu, Feb 06, 2020 at 11:51:16AM -0500, Peter Xu wrote: > > On Thu, Feb 06, 2020 at 08:28:18AM -0800, Sean Christopherson wrote: > > > On Thu, Feb 06, 2020 at 11:14:15AM -0500, Peter Xu wrote: > > > > On Tue, Jan 21, 2020 at 02:31:50PM -0800, Sean Christopherson wrote: > > > > > Move memslot deletion into its own routine so that the success path for > > > > > other memslot updates does not need to use kvm_free_memslot(), i.e. can > > > > > explicitly destroy the dirty bitmap when necessary. This paves the way > > > > > for dropping @dont from kvm_free_memslot(), i.e. all callers now pass > > > > > NULL for @dont. > > > > > > > > > > Add a comment above the code to make a copy of the existing memslot > > > > > prior to deletion, it is not at all obvious that the pointer will become > > > > > stale during sorting and/or installation of new memslots. > > > > > > > > Could you help explain a bit on this explicit comment? I can follow > > > > up with the patch itself which looks all correct to me, but I failed > > > > to catch what this extra comment wants to emphasize... > > > > > > It's tempting to write the code like this (I know, because I did it): > > > > > > if (!mem->memory_size) > > > return kvm_delete_memslot(kvm, mem, slot, as_id); > > > > > > new = *slot; > > > > > > Where @slot is a pointer to the memslot to be deleted. At first, second, > > > and third glances, this seems perfectly sane. > > > > > > The issue is that slot was pulled from struct kvm_memslots.memslots, e.g. > > > > > > slot = &slots->memslots[index]; > > > > > > Note that slots->memslots holds actual "struct kvm_memory_slot" objects, > > > not pointers to slots. When update_memslots() sorts the slots, it swaps > > > the actual slot objects, not pointers. I.e. after update_memslots(), even > > > though @slot points at the same address, it's could be pointing at a > > > different slot. As a result kvm_free_memslot() in kvm_delete_memslot() > > > will free the dirty page info and arch-specific points for some random > > > slot, not the intended slot, and will set npages=0 for that random slot. > > > > Ah I see, thanks. Another alternative is we move the "old = *slot" > > copy into kvm_delete_memslot(), which could be even clearer imo. > > The copy is also needed in __kvm_set_memory_region() for the MOVE case. Right. I actually meant to do all "old = *slot" in any function we need to cache the data explicitly, with that we also need another one after calling kvm_delete_memslot() for move. But with the comment as you suggested below it looks good to me too. Thanks, > > > However I'm not sure whether it's a good idea to drop the test-by for > > this. Considering that comment change should not affect it, would you > > mind enrich the comment into something like this (or anything better)? > > > > /* > > * Make a full copy of the old memslot, the pointer will become stale > > * when the memslots are re-sorted by update_memslots() in > > * kvm_delete_memslot(), while to make the kvm_free_memslot() work as > > * expected later on, we still need the cached memory slot. > > */ > > As above, it's more subtle than just the kvm_delete_memslot() case. > > /* > * Make a full copy of the old memslot, the pointer will become stale > * when the memslots are re-sorted by update_memslots() when deleting > * or moving a memslot, and additional modifications to the old memslot > * need to be made after calling update_memslots(). > */ > -- Peter Xu _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel