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Peter Anvin" , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Shuah Khan , Shuah Khan , Vishal Annapurve , Andrew Morton , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Youngjun Park , Qi Zheng , Shakeel Butt , Kiryl Shutsemau , Baoquan He , Jason Gunthorpe , Vlastimil Babka , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev References: <20260618-gmem-inplace-conversion-v8-0-9d2959357853@google.com> <20260618-gmem-inplace-conversion-v8-1-9d2959357853@google.com> Content-Language: en-US From: Binbin Wu In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/23/2026 9:37 AM, Sean Christopherson wrote: > On Mon, Jun 22, 2026, Binbin Wu wrote: >> On 6/19/2026 8:31 AM, Ackerley Tng via B4 Relay wrote: >> >> [...] >> >>> >>> +static u64 kvm_gmem_get_attributes(struct inode *inode, pgoff_t index) >>> +{ >>> + struct maple_tree *mt = &GMEM_I(inode)->attributes; >>> + void *entry = mtree_load(mt, index); >>> + >>> + return WARN_ON_ONCE(!entry) ? 0 : xa_to_value(entry); >> >> If the entry is unexpectedly missing, returning 0 means the attribute would >> be treated as shared. And then in kvm_gmem_fault_user_mapping(), it would >> allow the userspace to fault in the folio. >> >> Should gmem deny such edge case? > > After several bugs this year where a WARN_ON_ONCE() fired, but was entirely > insufficient to prevent true badness, I'm definitely senstive to making the "bad" > behavior as harmless as possible. > > However, in this case I think we're just hosed. If KVM treats the memory as > private, KVM will incorrectly do prepare(), incorrectly allow populate(), and > will caused missed invalidations (though I suppose __kvm_gmem_set_attributes() > "only" lies to userspace in that case). > > That said, assuming SHARED is definitely odd for cases where guest_memfd *can't* > hold shared memory. Ditto for assuming PRIVATE. Indeed. > What if we instead fall back to > the "init" state, e.g.? LGTM. > > static u64 kvm_gmem_get_attributes(struct inode *inode, pgoff_t index) > { > struct maple_tree *mt = &GMEM_I(inode)->attributes; > void *entry = mtree_load(mt, index); > > if (WARN_ON_ONCE(!entry)) { > bool shared = GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_INIT_SHARED; > > return shared ? 0 : KVM_MEMORY_ATTRIBUTE_PRIVATE; > } > > return xa_to_value(entry); > } >