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b=Sk36rrz1lHFTrwIzHYCFaYZmTlcopkOYJNutw9p7SwlvWgOwlo8LNwh7UGr/QrxGq Y/4rC/JdWWhNb/lWg69SqV9OzHX8ORX4UrblhHVXBDholvSyGoUmtPHH9lkZTgF8uZ zLiWOxTPx5zNMyeFxCuKfKD4qZ3A321buNLcNJo934wJUEbw1bCNe3tC6IOhUQI/F1 gS2xnR/9kUU1B5hZfNIr9fmHI/HP3m2c6PnC/iDDvZ1KQOqKmPpQFVhvqCd46WyTZ2 M0z+IqXjBFZcDdoma8BJwJZr8YekEnonRlLiO+L6RJLlPCwtUgV1H474XbPot4sZyu 7X9EBs2PB3Bqw== Date: Thu, 22 Jan 2026 12:57:50 +0200 From: Mike Rapoport To: Breno Leitao Cc: Alexander Graf , Pasha Tatashin , Pratyush Yadav , linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-mm@kvack.org, usamaarif642@gmail.com, rmikey@meta.com, clm@fb.com, riel@surriel.com, kernel-team@meta.com, SeongJae Park Subject: Re: [PATCH v4] kho: kexec-metadata: track previous kernel chain Message-ID: References: <20260121-kho-v4-1-5c8fe77b6804@debian.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260121-kho-v4-1-5c8fe77b6804@debian.org> X-Rspamd-Server: rspam11 X-Stat-Signature: ci4nnbueqnjqddecomukfnshbmibu7ho X-Rspam-User: X-Rspamd-Queue-Id: 4A10BA000B X-HE-Tag: 1769079479-76942 X-HE-Meta: U2FsdGVkX19cCMRkvKEBEi2LduDIrGnyudjMCkgHfcch957RQsdITwonKHjC3nN911mafzCB7+C2D4Q7LsKpRuRZ4tFf49dyUfAfQA3lImFtLCfhl5UK0xNyhlAvEiAEzPcYMq6k8Pvp2Sl+y85w/I2xjW1bBkvRdOY6JK6T1QAr2tNYZkgb7Ks9hwW+FuGfFvg0xqw9LpYbxrqXve00a8kO/Q9Kq1dc+Xs+w8YCqXnEaRZ97mqfkwvZS24ab2EhyR9k5sgUkD8HCpggOUiGKGaat1gx8alWYqFF5IIK9Boxr+k6bGiPMOrnu+Fy1UG99FEF77sx9SXlJTQHT0pZ4/AUDV9k/uUFiPzv7zyy+l8huPlptvGjvmXUlpNcEVTY3YUZTKFDt2pai08VPKnNAHQnwzwhA+LLmHEqb/b5zwHoVYs36l119vKUnTztzHgXhX4zHrxEwMwUnDxAjvP4ky+RYBug5GKRFlMYSTVh7zUmdZRjFHaNYA81GuH2qLkGo0txFqf6hindPvwHz0p0btOhoB0EN7SAoEVX84TzeNJtfj5flWSOygC91RZS3PVS7F3o2P9LrbdcFaXC4cjliMs8j6hd70NiovcmxuQMYNjgYQRtPERguQYrTckBQmzoSfoo7hB69bQWWdiS+f398mG9kDy45UDT7jIQZqfQbWqM8n0233WTYTTcB78mhZ4S/ylANyJJg9uLJOBoLXaAtuYZg1Xo49EFWhH5XvQzAYE1RsQzUzQ+ozIlCqA3XJLXO0zJ+3DZ8UpLfgCIYKEUs6UpFPMcgM4IppohNAO7RzQ+WJ21Piwy6VA6ZhDa4XRdzXpV5i9hGnS7HPUuHmtOVpmqhVznc9oXIR9IbxjSzGQx0jAfZUAnxEtPDqbE9diu5d9bBOg1MGnXR+LBd1ThZ2lL/GTWnphSm90Ud7OcqBCdDUnWoCPRBHouH7cx6AfAyR9yom/TFG07yDqmx1O nK7OPALZ vAFX0X6tArKKw9GInXL3lcpIBPsgc70FZAUtpHnTHsI8Rn9o+f+MfOOE5jd5SExjHejtgJJUMQgo2eGD2HfFI9cyevSqP+Pq1ZCUKT3D4DQxirmvwO2ZXss39EHgnNzKN1y9B7E2PBcCcsTbOodxsOT1LVsNvUjHHlUolcTwMK5bru6V3bSMZf6LA7XV8PyXLsIQQtWtcgefS/iCZ55WLAxAWUghVSNYbiFbGxS8Mjq5VZjlMZKZUAl/1pp/zf8MSaNxiPqYHMrFQISkqtevITYIOh5isjf9zdIGM2bACjcIm05+I0Y/c2zBw5szHSdKV4Inl81TT5WExqx/DzqYAlYcEIBav2PYTx7XGBBi2dxZQhzsEaHbxY86MpeL6/tErSBCp X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Breno, On Wed, Jan 21, 2026 at 06:50:38AM -0800, Breno Leitao wrote: > Use Kexec Handover (KHO) to pass the previous kernel's version string > and the number of kexec reboots since the last cold boot to the next > kernel, and print it at boot time. > > Example output: > [ 0.000000] KHO: exec from: 6.19.0-rc4-next-20260107 (count 1) > > Motivation > ========== > > Bugs that only reproduce when kexecing from specific kernel versions > are difficult to diagnose. These issues occur when a buggy kernel > kexecs into a new kernel, with the bug manifesting only in the second > kernel. > > Recent examples include the following commits: > > * eb2266312507 ("x86/boot: Fix page table access in 5-level to 4-level paging transition") > * 77d48d39e991 ("efistub/tpm: Use ACPI reclaim memory for event log to avoid corruption") > * 64b45dd46e15 ("x86/efi: skip memattr table on kexec boot") > > As kexec-based reboots become more common, these version-dependent bugs > are appearing more frequently. At scale, correlating crashes to the > previous kernel version is challenging, especially when issues only > occur in specific transition scenarios. > > Implementation > ============== > > The kexec metadata is stored as a plain C struct (struct kho_kexec_metadata) > rather than FDT format, for simplicity and direct field access. It is > registered via kho_add_subtree() as a separate subtree, keeping it > independent from the core KHO ABI. This design choice: > > - Keeps the core KHO ABI minimal and stable > - Allows the metadata format to evolve independently > - Avoids requiring version bumps for all KHO consumers (LUO, etc.) > when the metadata format changes > > The struct kho_metadata contains two fields: > - previous_release: The kernel version that initiated the kexec > - kexec_count: Number of kexec boots since last cold boot > > On cold boot, kexec_count starts at 0 and increments with each kexec. > The count helps identify issues that only manifest after multiple > consecutive kexec reboots. > > Signed-off-by: Breno Leitao > Acked-by: SeongJae Park > --- > Changes in v4: > - Squashed everything in a single commit > - Moved from FDT to C structs (Pratyush) > - Usage of subtress intead of FDT directly (Pratyush) > - Renamed a bunch of variables and functions. > - Link to v3: https://patch.msgid.link/20260108-kho-v3-0-b1d6b7a89342@debian.org > > Changes in v3: > - Remove the extra CONFIG for this feature. > - Reworded some identifiers, properties and printks. > - Better documented the questions raised during v2. > - Link to v2: https://patch.msgid.link/20260102-kho-v2-0-1747b1a3a1d6@debian.org > > Changes from v2 to v1 (RFC) > - Track the number of kexecs since cold boot (Pasha) > - Change the printk() order compared to KHO > - Rewording of the commit summary > - Link to RFC: https://patch.msgid.link/20251230-kho-v1-1-4d795a24da9e@debian.org > --- > include/linux/kho/abi/kexec_handover.h | 29 +++++++++++++++ > kernel/liveupdate/kexec_handover.c | 65 ++++++++++++++++++++++++++++++++++ > 2 files changed, 94 insertions(+) > > diff --git a/include/linux/kho/abi/kexec_handover.h b/include/linux/kho/abi/kexec_handover.h > index 285eda8a36e45..e18022a4e664d 100644 > --- a/include/linux/kho/abi/kexec_handover.h > +++ b/include/linux/kho/abi/kexec_handover.h > @@ -11,6 +11,7 @@ > #define _LINUX_KHO_ABI_KEXEC_HANDOVER_H > > #include > +#include > > /** > * DOC: Kexec Handover ABI > @@ -84,6 +85,34 @@ > /* The FDT property for sub-FDTs. */ > #define KHO_FDT_SUB_TREE_PROP_NAME "fdt" > > +/** > + * DOC: Kexec Metadata ABI > + * It would be nice to link it from Documentation/ as well ;-) > + * The "kexec-metadata" subtree stores optional metadata about the kexec chain. > + * It is registered via kho_add_subtree(), keeping it independent from the core > + * KHO ABI. This allows the metadata format to evolve without affecting other > + * KHO consumers. > + * > + * The metadata is stored as a plain C struct rather than FDT format for > + * simplicity and direct field access. > + */ > + > +/** > + * struct kho_kexec_metadata - Kexec metadata passed between kernels > + * @previous_release: Kernel version string that initiated the kexec > + * @kexec_count: Number of kexec boots since last cold boot > + * > + * This structure is preserved across kexec and allows the new kernel to > + * identify which kernel it was booted from and how many kexec reboots > + * have occurred. > + */ > +struct kho_kexec_metadata { > + char previous_release[__NEW_UTS_LEN + 1]; > + u32 kexec_count; > +} __packed; > + > +#define KHO_METADATA_NODE_NAME "kexec-metadata" > + > /** > * DOC: Kexec Handover ABI for vmalloc Preservation > * > diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c ... > static __init int kho_init(void) > { > const void *fdt = kho_get_fdt(); > @@ -1357,6 +1413,15 @@ static __init int kho_init(void) > if (err) > goto err_free_fdt; > > + if (fdt) > + kho_process_kexec_metadata(); Can't we move it into the existing if (fdt) below? > + > + /* Populate kexec metadata for the possible next kexec */ > + err = kho_populate_kexec_metadata(); > + if (err) > + pr_warn("failed to initialize kexec-metadata subtree: %d\n", > + err); Please follow if (err) goto err_ pattern. kho_populate_kexec_metadata() failure essentially means that we failed to allocate memory. This shouldn't happen that early in boot, but if it did, then something is utterly wrong. > + > if (fdt) { > kho_in_debugfs_init(&kho_in.dbg, fdt); > return 0; -- Sincerely yours, Mike.