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[70.24.86.62]) by smtp.gmail.com with ESMTPSA id o15-20020a05620a15cf00b0075784a8f13csm937274qkm.96.2023.05.17.15.20.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 17 May 2023 15:20:36 -0700 (PDT) Date: Wed, 17 May 2023 18:20:34 -0400 From: Peter Xu To: James Houghton Cc: Axel Rasmussen , Alexander Viro , Andrew Morton , Christian Brauner , David Hildenbrand , Hongchen Zhang , Huang Ying , "Liam R. Howlett" , Miaohe Lin , "Mike Rapoport (IBM)" , Nadav Amit , Naoya Horiguchi , Shuah Khan , ZhangPeng , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Anish Moorthy Subject: Re: [PATCH 1/3] mm: userfaultfd: add new UFFDIO_SIGBUS ioctl Message-ID: References: <20230511182426.1898675-1-axelrasmussen@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, May 17, 2023 at 06:12:33PM -0400, Peter Xu wrote: > On Thu, May 11, 2023 at 03:00:09PM -0700, James Houghton wrote: > > On Thu, May 11, 2023 at 11:24 AM Axel Rasmussen > > wrote: > > > > > > So the basic way to use this new feature is: > > > > > > - On the new host, the guest's memory is registered with userfaultfd, in > > > either MISSING or MINOR mode (doesn't really matter for this purpose). > > > - On any first access, we get a userfaultfd event. At this point we can > > > communicate with the old host to find out if the page was poisoned. > > > - If so, we can respond with a UFFDIO_SIGBUS - this places a swap marker > > > so any future accesses will SIGBUS. Because the pte is now "present", > > > future accesses won't generate more userfaultfd events, they'll just > > > SIGBUS directly. > > > > I want to clarify the SIGBUS mechanism here when KVM is involved, > > keeping in mind that we need to be able to inject an MCE into the > > guest for this to be useful. > > > > 1. vCPU gets an EPT violation --> KVM attempts GUP. > > 2. GUP finds a PTE_MARKER_UFFD_SIGBUS and returns VM_FAULT_SIGBUS. > > 3. KVM finds that GUP failed and returns -EFAULT. > > > > This is different than if GUP found poison, in which case KVM will > > actually queue up a SIGBUS *containing the address of the fault*, and > > userspace can use it to inject an appropriate MCE into the guest. With > > UFFDIO_SIGBUS, we are missing the address! > > > > I see three options: > > 1. Make KVM_RUN queue up a signal for any VM_FAULT_SIGBUS. I think > > this is pointless. > > 2. Don't have UFFDIO_SIGBUS install a PTE entry, but instead have a > > UFFDIO_WAKE_MODE_SIGBUS, where upon waking, we return VM_FAULT_SIGBUS > > instead of VM_FAULT_RETRY. We will keep getting userfaults on repeated > > accesses, just like how we get repeated signals for real poison. > > 3. Use this in conjunction with the additional KVM EFAULT info that > > Anish proposed (the first part of [1]). > > > > I think option 3 is fine. :) > > Or... option 4) just to use either MADV_HWPOISON or hwpoison-inject? :) I just remember Axel mentioned this in the commit message, and just in case this is why option 4) was ruled out: They expect that once poisoned, pages can never become "un-poisoned". So, when we live migrate the VM, we need to preserve the poisoned status of these pages. Just to supplement on this point: we do have unpoison (echoing to "debug/hwpoison/hwpoison_unpoison"), or am I wrong? > > Besides what James mentioned on "missing addr", I didn't quickly see what's > the major difference comparing to the old hwpoison injection methods even > without the addr requirement. If we want the addr for MCE then it's more of > a question to ask. > > I also didn't quickly see why for whatever new way to inject a pte error we > need to have it registered with uffd. Could it be something like > MADV_PGERR (even if MADV_HWPOISON won't suffice) so you can inject even > without an userfault context (but still usable when uffd registered)? > > And it'll be alawys nice to have a cover letter too (if there'll be a new > version) explaining the bits. > > Thanks, > > -- > Peter Xu -- Peter Xu