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b=LIVACHG1RijgMjCcG/Za6quG10uP/bwvgH9QE8LBE1Ivatgc9Gsxomr3AtpNUFPU0 A+6txV+JhFA/FQ9wi7C+7pWYpZ0e+Pc7WqKMe9jkABrVblw1ZFcwlZAztCqgB1P/e3 p9b8wN4tDVitUGLc/V+1hQT1NZFTVD7m3WdPU0oqKQlNmHSSGk9l9hK3CeTbNDWqSe VN3sAKFewBC6xe0t5rYtB+sbuiw5e5lDQoKvIRxdjJ0DrtoKis23WnQwx7fOelIVvL Atltab4Pstjrs4D4dYuIJSyaVZnuHq7QWlD07iE6RtqSOvkxEaO4SzTr/vUKZm0prA whzCKqWsehupA== From: Pratyush Yadav To: David Matlack Cc: Pasha Tatashin , tarunsahu@google.com, Andrew Morton , joro@8bytes.org, Mike Rapoport , Pratyush Yadav , maz@kernel.org, arnd@arndb.de, jgg@nvidia.com, robin.murphy@arm.com, loganodell@google.com, oupton@kernel.org, kevin.tian@intel.com, alex@shazbot.org, dwmw2@infradead.org, pbonzini@redhat.com, seanjc@google.com, will@kernel.org, Alexander Graf , Bjorn Helgaas , baolu.lu@linux.intel.com, linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, sagis@google.com, linux-mm@kvack.org, kvmarm@lists.linux.dev, iommu@lists.linux.dev, linux-pci@vger.kernel.org, kexec@lists.infradead.org, skhawaja@google.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v2 3/4] liveupdate: Replace compatibility string with a 2-bit pair feature header In-Reply-To: (David Matlack's message of "Mon, 26 Oct 2026 20:49:02 +0000") References: Date: Wed, 28 Oct 2026 18:17:02 +0100 Message-ID: <2vxz8q3hoicx.fsf@kernel.org> User-Agent: Gnus/5.13 (Gnus v5.13) X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 3B25140002 X-Stat-Signature: 4utx3un5arer5h737qo4hopeoq1phedp X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-Rspam-User: X-HE-Tag: 1793207830-849341 X-HE-Meta: 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 7cj1MJBn +xAcBkJAt5qIE+nIM4gP8H+go0MauPuXy3VEp6C2foXHPskQdvxajSjZMSSp3/JVJ+/5ExKHlaZpKYKs0VgtxSib0UrJ6h22vRsKsKMxnEOHyN3i2gMAUUBiYX6dAQ8L7yN77uBdzWwTvJRCjVecz8+qLQqcq/GQdYk2MJncaB3+zyM8iioHxzHvQlmCF7X84xWzRFvgIC/R0VDAo8IsnZp51BTXDYiftzPstjeD2fg9zoPMYP5lK2cEmVA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Mon, Oct 26 2026, David Matlack wrote: > On 2026-10-25 04:38 PM, Pasha Tatashin wrote: >> On Fri, Oct 23, 2026 at 9:50=E2=80=AFAM David Matlack wrote: > [...] >> The reason I care about keeping the header compact is that we are >> defining a foundational kernel ABI and want to use this header across >> almost every serialized data structure so we can evolve them later >> without breaking compatibility. While some top-level structs are on >> sparse folios today, other serialized structs are tightly packed, and >> we are looking to add KHO slab support in the future where tightly >> packed allocations will be common. > > My intuition is we will have a small number of headers per preserved > file and per preserved FLB. One for the top-level struct and then > optionally a few more for sub-structs like arch-specific data. That is > why I am less worried 8 bytes per header, even if we fix the folio > problem. > > Maybe where our visions differ is when there are collections of > serialized structs, like KHO blocks. For collections I don't think the > elements will each have their own headers. Otherwise the elements could > be different sizes. kho_block today doesn't store entry_size in the serialized payload. It relies on its caller to do so. With extensible structures, this can get messy. Sure, the owner of the kho_block_set can track the size somewhere in its payload, but perhaps we should store the size in kho_block_header_ser to begin with? Anyway, not a problem this series needs to solve, but something we should eventually do. [...] --=20 Regards, Pratyush Yadav