From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=209.85.216.47 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792624699; b=h1n76a6ngHxCwLg3KaccbGoNFVVbdD6QFnznU6m/28xmHoFunzRiQtPiM6mTby/Bj/vF C//IlkJdlxlZ5mSNGAmMbadSCPDe7uoBmLXH4vwUfP4gIp+7mQmqRX3slcHNCy1qPQ4yV mhtCKapacCF5c+mcceGVNjNN2Wu4W79nsam4Ej/AaT7FXOE72CtpKPc/2GqhKYR3t/qiA a8/lRIiSbaGk9EorgTQjRAR29gRl195ZniFBpeyOrAUcCnIfxtKvdG7zh1xPthzJ9u3TZ BbpZ263B62BT7BG6T35v+b1hFuZE/pdrZjY3KJ3lOAdGRQo8vB+bxWG6YTjvFMNnprQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792624699; h=DMARC-Filter:Received:DKIM-Signature:X-Google-DKIM-Signature: X-Forwarded-Encrypted:X-Gm-Message-State:X-Gm-Gg:X-Received:Received: Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:Content-Transfer-Encoding: In-Reply-To; bh=7nCAo4+9lz8Iyt19nkj9TmOxKkMhE+NTyi1XeRL1MPk=; b=cXYYR0ayplA52lcEsmNjrt5+EEUjW/1rOsHetFed7lQkcgHqeERM2iY/yLQ92vsz5SIQ Vf6OkQVdFPOqBRXxSKg7peVGYofE4qzrwtZZE1vrEaFtZZGGx029/vy2GUHjo4YO815nH QQKDEEhzDSFrIO5qmt4WyA0yKMrGn3uavPfAjH8f4G+6+Cb4J5vhGrCI5xSjAsXnUyoJB 7Y3Vx3N2BoTHuj6bmAyO6L5f96ZKPtgkstVWhi9rJBYuz7VMsm/D+4kGSwbXvijI1jgax 9BNsAemqJzri4nEOr3ttdXxNZcd/8Qf3J0twk/Yi46GSN6reBASrRi/Y9QffoVt3OCw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=google.com; spf=pass smtp.mailfrom=google.com; dkim=pass header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20251104 header.b=XTgC5U1a; arc=none smtp.remote-ip=209.85.216.47 Received: from mail-pj1-f47.google.com (mail-pj1-f47.google.com [209.85.216.47]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 358C0606A31F for ; Wed, 21 Oct 2026 23:18:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 358C0606A31F Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20251104 header.b=XTgC5U1a Received: by mail-pj1-f47.google.com with SMTP id 98e67ed59e1d1-3ac77270fbfso680742a91.2 for ; Wed, 21 Oct 2026 16:18:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1792624698; x=1793229498; darn=vger.kernel.org; h=in-reply-to:content-transfer-encoding:content-disposition :content-type:mime-version:references:message-id:subject:cc:to:from :date:from:to:cc:subject:date:message-id:reply-to:content-type; bh=7nCAo4+9lz8Iyt19nkj9TmOxKkMhE+NTyi1XeRL1MPk=; b=XTgC5U1avhZC7kS60Wfhcu4iNJ6NZUTB15w7iAv0P+4Xk0fU3Z9lqDMg0+5uT/ZioR V1pzlNVAVaG1PI44ziVmRwdXwwPNLl1i1fbe01hLsNEj75SM+9vlW/bhpCwsK76fEi41 m2yoJJDX2Z6BXGc/sMzdvX0/s/ciDTn2uTVTOxDwdfShxa/IGvCrC/6KhBLS/TI0AaOK 4FSqZaRMDg7fPIneOSeherAD3s0ms3oLr8jLx84C775gRsQ0tKCcLrtO9Ynuwjr/HLV3 EF5B/Fp6LB+jA5yziNZjR3V+KQJcowR4G7b1+lHp542RRnZ0iuI6AhGdOkKSyHF4sZbp O56w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1792624698; x=1793229498; h=in-reply-to:content-transfer-encoding:content-disposition :content-type:mime-version:references:message-id:subject:cc:to:from :date:x-gm-gg:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to:content-type; bh=7nCAo4+9lz8Iyt19nkj9TmOxKkMhE+NTyi1XeRL1MPk=; b=neV2CakQ2t1TcfyAm0Acbsp1JHDXrK6aLV/cxCE4jj8Gr3sYqniRwcSLyCRtj97Z+F tBcM1wWpzCJk+90+qOZnQW3PAg3qPlw9tCH2trDICMck3Kq+m2dw+5FVWxn37QjOzLiz ssOEf35WLCURZtn0zmDTnducGwocIlNASsN2RgVzprS7dWvDvH0Lyp/0GlhQ8ZahMp5O QbTnelbEz2d5D9L+axAz0pUZxbZU7ooH7V9yJ/GWk+mXlYvlIbFaCTdCljLI3JFxWDVp taCLYFmL86oBWlBZPMvi0h4f8RBiNMSC/Qa2O3l+q+fECKvCBR21zar1vxW8Jp8Orv/T uKSQ== X-Forwarded-Encrypted: i=1; AKwUvBwViYp95TEydAxY3GETWLJEqc9uAPnk+XWp/2TsheYE4rptVP6NuELaY64PoqKv3JR4qjbV8mDun3v1@vger.kernel.org X-Gm-Message-State: AFq9FYK0mFW6kJneSW9MdXUcodfXYuRd05bawJRut9iFtVrVVnDGoWnr rEc6AIts/Lo2vaOcrwGzaVFOwh/X0TVYHAGsdIe8LWfcAMECrWUlFYiCGBA2HHVrRA== X-Gm-Gg: AYBFou3hLxzzTZ/fEAnnBQln7k2faUSke1ELoe2J88OOCvOmGCjw4Fj6XUPKyhLnV+X lTycEEGEFc3zHwrtvV+qRkFZWbcHyX3stQYiBETSQqAkoo4qpbL07GzWE5cFlvFrbbCLNUw1XT0 r6V0fpjR0HROWqxHVj4uhBewawj6XUC9IFviuhNIYJ8lCUSVLBmXD51V7nNfihFx0UVKqcoTUu7 uvAauhDhOt9hnyJPaB6thGEKjbiuhhRkbUfnrdFVyA+KACtbS3zB6BSiLMjO0u5SZy7fcjc/3Qd ifvmE0m3SG5h63aVQZHc6gvm7eie11NgnKBZhcgHD8z/v/39ITSGIHdRRPZd+mitu0l+Rd0ZZXA uw9yuMtMknUETIQZkbfUWlZKQ/buKNVmVHBJyH1QJ2AWSZ0UydVUe9llRCCKAX7Aqxj/4S19ihI OBj6aErq/qW2X60bE5gK9j13NEn+ol/7F6wbbdV7gVnfnxLn9xKhS7L72xl/Xdl35kGaIyCcK7A 7UXsbliL5NtLFotsTQ2pt+EcW55NTPDcUzeevt843ytEhDT9qdI/BQ1ryZqp6bMytuEOYmW18eK 2IHcZR1L16DpxcNCNY6rG3Utt+us8TR0NeELDw== X-Received: by 2002:a17:90b:3c8d:b0:3ac:7f4c:a690 with SMTP id 98e67ed59e1d1-3ad86eeb9e6mr2442353a91.35.1792624697709; Wed, 21 Oct 2026 16:18:17 -0700 (PDT) Received: from google.com (64.75.127.34.bc.googleusercontent.com. [34.127.75.64]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3ad85f62d22sm3123694a91.4.2026.10.21.16.18.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Oct 2026 16:18:16 -0700 (PDT) Date: Wed, 21 Oct 2026 23:18:13 +0000 From: David Matlack To: Pasha Tatashin Cc: Tarun Sahu , 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 4/4] mm: memfd_luo: Add the 2-bit pair feature header Message-ID: References: <20261021152527.375070-1-tarunsahu@google.com> <20261021152527.375070-5-tarunsahu@google.com> X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On 2026-10-21 01:38 PM, Pasha Tatashin wrote: > On Wed, Oct 21, 2026 at 11:25 AM Tarun Sahu wrote: > > > > Embed the fixed-size 128-byte struct luo_feature_hdr at the start of > > struct memfd_luo_ser and define enum memfd_luo_feature > > (MEMFD_LUO_FEAT_CORE and MEMFD_LUO_FEAT_MAX) for memfd preservation. > > > > All current struct memfd_luo_ser fields (pos, size, seals, flags, > > nr_folios, folios) and MEMFD_LUO_ALL_SEALS belong to > > MEMFD_LUO_FEAT_CORE (LUO_FEAT_REQUIRED in memfd_luo_features). Future > > changes add new entries to enum memfd_luo_feature before > > MEMFD_LUO_FEAT_MAX and append any new fields to the end of struct > > memfd_luo_ser so existing field offsets never shift. > > > > Signed-off-by: Logan Odell > > Signed-off-by: Tarun Sahu > > --- > > include/linux/kho/abi/memfd.h | 35 ++++++++++++++++++++++++++++++----- > > mm/memfd_luo.c | 14 ++++++++++++++ > > 2 files changed, 44 insertions(+), 5 deletions(-) > > > > diff --git a/include/linux/kho/abi/memfd.h b/include/linux/kho/abi/memfd.h > > index 371ffd108a79..f0b5235a408d 100644 > > --- a/include/linux/kho/abi/memfd.h > > +++ b/include/linux/kho/abi/memfd.h > > @@ -13,6 +13,7 @@ > > > > #include > > #include > > +#include > > > > /** > > * DOC: memfd Live Update ABI > > @@ -21,11 +22,20 @@ > > * reboot using the LUO. > > * > > * The state is serialized into a packed structure `struct memfd_luo_ser` > > - * which is handed over to the next kernel via the KHO mechanism. > > + * which is handed over to the target kernel via the KHO mechanism. > > * > > - * This interface is a contract. Any modification to the structure layout > > - * constitutes a breaking change. Such changes require incrementing the > > - * version number in the MEMFD_LUO_FH_COMPATIBLE string. > > + * This interface is a contract. `struct memfd_luo_ser` starts with a fixed > > + * 128-byte `struct luo_feature_hdr`. The relative order and offsets of > > + * fields must remain unchanged as features are added or retired: new fields > > + * are appended at the end of `struct memfd_luo_ser` under a new feature > > + * flag in `enum memfd_luo_feature` (before `MEMFD_LUO_FEAT_MAX`), and > > + * retired fields are kept in place so subsequent offsets never shift. In > > + * the target kernel, fields of a `LUO_FEAT_REQUIRED` feature are guaranteed > > + * present once `luo_feature_hdr_validate()` succeeds, whereas fields of a > > + * `LUO_FEAT_OPTIONAL` feature should be accessed only after checking > > + * `luo_feature_is_active()`. All current fields (`pos`, `size`, `seals`, > > + * `flags`, `nr_folios`, `folios`) and `MEMFD_LUO_ALL_SEALS` belong to > > + * `MEMFD_LUO_FEAT_CORE` (`LUO_FEAT_REQUIRED` in `memfd_luo_features`). > > */ > > > > /** > > @@ -58,7 +68,7 @@ struct memfd_luo_folio_ser { > > > > /* > > * The set of seals this version supports preserving. If support for any new > > - * seals is needed, add it here and bump version. > > + * seals is needed, gate them behind a new feature flag. > > */ > > #define MEMFD_LUO_ALL_SEALS (F_SEAL_SEAL | \ > > F_SEAL_SHRINK | \ > > @@ -67,8 +77,22 @@ struct memfd_luo_folio_ser { > > F_SEAL_FUTURE_WRITE | \ > > F_SEAL_EXEC) > > > > +/** > > + * enum memfd_luo_feature - Feature indices for &struct memfd_luo_ser. > > + * @MEMFD_LUO_FEAT_CORE: Core feature covering `pos`, `size`, `seals`, `flags`, > > + * `nr_folios`, `folios`, and %MEMFD_LUO_ALL_SEALS. > > + * @MEMFD_LUO_FEAT_MAX: Number of defined &enum memfd_luo_feature features. > > + */ > > +enum memfd_luo_feature { > > + MEMFD_LUO_FEAT_CORE = 0, > > + MEMFD_LUO_FEAT_MAX, > > +}; > > + > > +static_assert((int)MEMFD_LUO_FEAT_MAX <= LUO_FEAT_MAX); > > + > > /** > > * struct memfd_luo_ser - Main serialization structure for a memfd. > > + * @features: ABI feature and size header for memfd_luo_ser. > > * @pos: The file's current position (f_pos). > > * @size: The total size of the file in bytes (i_size). > > * @seals: The seals present on the memfd. The seals are uABI so it is safe > > @@ -79,6 +103,7 @@ struct memfd_luo_folio_ser { > > * struct memfd_luo_folio_ser. > > */ > > struct memfd_luo_ser { > > + struct luo_feature_hdr features; > > I have not looked into this yet, but I would prefer to use some kind > of macro constructor for serialized structs. That way, they will > automatically include the header, making it easier to search the code > for what is actually preserved versus what exists only in this kernel. If you want to know what is preserved don't you just look at include/linux/kho/abi? > > So, I think something like this: > /* Base serialization header: 4 bytes total size, 4 bytes flags */ > struct luo_ser_hdr { > __u32 total_size; Why do we need the size? I'm not really opposed to it I just don't understand what problem it solves. The luo_ser_hdr is needed for top-level file handler and FLB ser structs. Those structs are always dynamically allocated and preserved with KHO. On retrieve, KHO gives you back the blob of memory. You look at the header to determine what features are used in the struct, and then access the fields associated with those features. At no point do you need to check the size. The only time you need the size is for a KHO block array. You want to store the size of the KHO block array elements so that the retrieving kernel can correctly initialize the iterator. e.g. from the PCI base series: /** * struct pci_ser - PCI Subsystem Live Update State * * This struct tracks state about all devices that are being preserved across * a Live Update for the next kernel. It contains only state owned by the PCI * core; the state a driver needs to resume its device is preserved separately * by that driver. * * @required_flags: Features that the next kernel must support to take over * the preserved devices. No flags are currently defined. * @optional_flags: Features that the next kernel may ignore if it does not * support them. No flags are currently defined. * @dev_ser_size: The size in bytes of each struct pci_dev_ser in @devices. * @nr_devices: The number of devices that were preserved. * @devices: Physical address of the first KHO block containing pci_dev_ser. */ struct pci_ser { u64 required_flags; u64 optional_flags; u32 dev_ser_size; <========== u32 nr_devices; u64 devices; }; > __u32 flags; > }; > > #define LUO_SER_INIT_HDR(obj, _flags) do { \ > (obj)->hdr.total_size = sizeof(*(obj)); \ > (obj)->hdr.flags = (_flags); \ > } while (0) > > #define DECLARE_LUO_SER(name, expected_size, members) \ > struct name { \ > struct luo_ser_hdr hdr; \ > members \ > }; \ > static_assert(sizeof(struct name) == (expected_size), \ > "LUO struct " #name " size mismatch!") Why? Practically every change to a serialized struct is going to involve appending one or more fields to the end of the struct. Asserting the size means we have to update the hard-coded size as well every time we do that. Is this to protect against embedded structs accidentally changing? > > Usage: > /* > * Header: 8 bytes (hdr: 4 bytes total_size + 4 bytes flags) > * Payload: 8 (pos) + 8 (size) + 4 (seals) + 4 (_pad) = 24 bytes > * Total: 32 bytes > */ > DECLARE_LUO_SER(memfd_luo_ser, 32, > __u64 pos; > __u64 size; > __u32 seals; > __u32 _pad; > ); IMO this makes the code harder to read. I think open coding keeps things simpler. struct pci_ser { struct luo_ser_hdr header; __u32 dev_ser_size; __u32 nr_devices; __u64 devices; }; LUO_SER_ASSSERT_SIZE(pci_ser, 32); > > struct memfd_luo_ser *ser; > ... > LUO_SER_INIT_HDR(ser, 0); > > Thanks, > Pasha > > > u64 pos; > > u64 size; > > u32 seals; > > diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c > > index 64f8b8cb75bd..ea1f4c8cd259 100644 > > --- a/mm/memfd_luo.c > > +++ b/mm/memfd_luo.c > > @@ -84,6 +84,10 @@ > > > > #include "internal.h" > > > > +static const u8 memfd_luo_features[MEMFD_LUO_FEAT_MAX] = { > > + [MEMFD_LUO_FEAT_CORE] = LUO_FEAT_REQUIRED, > > +}; > > + > > static int memfd_luo_preserve_folios(struct file *file, > > struct kho_vmalloc *kho_vmalloc, > > struct memfd_luo_folio_ser **out_folios_ser, > > @@ -284,6 +288,8 @@ static int memfd_luo_preserve(struct liveupdate_file_op_args *args) > > goto err_free_ser; > > } > > > > + luo_feature_hdr_init(&ser->features, sizeof(*ser), > > + memfd_luo_features); > > ser->pos = args->file->f_pos; > > inode_size = i_size_read(inode); > > > > @@ -525,6 +531,14 @@ static int memfd_luo_retrieve(struct liveupdate_file_op_args *args) > > if (!ser) > > return -EINVAL; > > > > + err = luo_feature_hdr_validate(&ser->features, memfd_luo_features); > > + if (err) { > > + pr_err("memfd_luo_ser feature header validation failed (size=%llu, flags[0]=%#llx): %pe\n", > > + ser->features.size, ser->features.flags[0], > > + ERR_PTR(err)); > > + goto free_ser; > > + } > > + > > /* Make sure the file only has seals supported by this version. */ > > if (ser->seals & ~MEMFD_LUO_ALL_SEALS) { > > err = -EOPNOTSUPP; > > -- > > 2.56.0.212.g129ede20d9-goog > >