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[35.199.49.16]) by smtp.gmail.com with ESMTPSA id 71dfb90a1353d-5eaca7cc59asm5934172e0c.5.2026.10.20.07.30.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Oct 2026 07:30:09 -0700 (PDT) Date: Tue, 20 Oct 2026 10:30:08 -0400 From: Pasha Tatashin To: Pratyush Yadav Cc: Pasha Tatashin , linux-kselftest@vger.kernel.org, rppt@kernel.org, shuah@kernel.org, akpm@linux-foundation.org, linux-mm@kvack.org, vipinsh@google.com, linux-kernel@vger.kernel.org, dmatlack@google.com, kexec@lists.infradead.org, graf@amazon.com Subject: Re: [PATCH v1 1/7] liveupdate: Add LIVEUPDATE_SESSION_FREEZE ioctl to freeze sessions early Message-ID: References: <20261015093816.2438942-1-pasha.tatashin@soleen.com> <20261015093816.2438942-2-pasha.tatashin@soleen.com> <2vxz8q3shboh.fsf@kernel.org> X-Mailing-List: linux-kernel@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=us-ascii Content-Disposition: inline In-Reply-To: <2vxz8q3shboh.fsf@kernel.org> On 10-20 13:08, Pratyush Yadav wrote: > Hi, > > On Thu, Oct 15 2026, Pasha Tatashin wrote: > > > Currently, LUO sessions are frozen automatically only inside the > > reboot(LINUX_REBOOT_CMD_KEXEC) pathway, where luo_session_serialize() > > iterates through all outgoing sessions under global write locks and > > freezes them serially during shutdown. Allowing userspace to decide when > > to freeze a session once it is done with it (and to freeze multiple > > sessions in parallel, since luo_session_freeze() only acquires the > > per-session mutex) avoids postponing this work to kexec shutdown and > > reduces blackout time. > > > > Introduce LIVEUPDATE_SESSION_FREEZE on session file descriptors to freeze > > a session early. The ioctl runs .freeze() across all preserved files in the > > session and serializes their metadata. Once frozen, no additional files can > > be added to the session, and subsequent freeze calls return -EBUSY. Freeze > > is only valid on outgoing sessions and returns -EINVAL on incoming ones. > > > > During kexec, sessions that were already frozen early are skipped, while > > sessions that did not invoke this ioctl auto-freeze as before. If an > > early-frozen session is released before kexec, its files are rolled back > > via .unfreeze() before being unpreserved. > > > > Signed-off-by: Pasha Tatashin > [...] > > @@ -487,8 +488,10 @@ int luo_file_freeze(struct luo_file_set *file_set, > > file_ser->token = luo_file->token; > > } > > > > - file_set_ser->count = file_set->count; > > - file_set_ser->files = kho_block_set_head_pa(&file_set->block_set); > > + if (file_set_ser) { > > + file_set_ser->count = file_set->count; > > + file_set_ser->files = kho_block_set_head_pa(&file_set->block_set); > > + } > > > > return 0; > > > > @@ -501,7 +504,7 @@ int luo_file_freeze(struct luo_file_set *file_set, > > /** > > * luo_file_unfreeze - Unfreezes all files in a file_set and clear serialization > > * @file_set: The file_set whose files are to be unfrozen. > > - * @file_set_ser: Serialized file_set. > > + * @file_set_ser: Serialized file_set, or %NULL if not serialized yet. > > * > > * This function rolls back the state of all files in a file_set after the > > * freeze phase has begun but must be aborted. It is the counterpart to > > @@ -512,7 +515,8 @@ int luo_file_freeze(struct luo_file_set *file_set, > > * their respective .unfreeze() handler callbacks. > > * > > * Context: This is called when the live update is aborted during > > - * the reboot() syscall, after luo_file_freeze() has been called. > > + * the reboot() syscall, after luo_file_freeze() has been called, > > + * or when an early-frozen session is released before kexec. > > */ > > void luo_file_unfreeze(struct luo_file_set *file_set, > > struct luo_file_set_ser *file_set_ser) > > @@ -521,7 +525,8 @@ void luo_file_unfreeze(struct luo_file_set *file_set, > > return; > > > > __luo_file_unfreeze(file_set, NULL); > > - memset(file_set_ser, 0, sizeof(*file_set_ser)); > > + if (file_set_ser) > > + memset(file_set_ser, 0, sizeof(*file_set_ser)); > > } > > > > /** > > diff --git a/kernel/liveupdate/luo_internal.h b/kernel/liveupdate/luo_internal.h > > index 64879ffe73781..b42369c3b1f30 100644 > > --- a/kernel/liveupdate/luo_internal.h > > +++ b/kernel/liveupdate/luo_internal.h > > @@ -64,6 +64,8 @@ struct luo_file_set { > > * previous kernel) sessions. > > * @retrieved: A boolean flag indicating whether this session has been > > * retrieved by a consumer in the new kernel. > > + * @frozen: A boolean flag indicating whether this session has been > > + * frozen early from userspace. > > * @file_set: A set of files that belong to this session. > > * @mutex: protects fields in the luo_session. > > */ > > @@ -71,6 +73,7 @@ struct luo_session { > > char name[LIVEUPDATE_SESSION_NAME_LENGTH]; > > struct list_head list; > > bool retrieved; > > + bool frozen; > > struct luo_file_set file_set; > > struct mutex mutex; > > }; > > diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c > > index f48e9a4185f97..403906937bc0f 100644 > > --- a/kernel/liveupdate/luo_session.c > > +++ b/kernel/liveupdate/luo_session.c > > @@ -34,10 +34,11 @@ > > * 1. Creation: A userspace agent calls `luo_session_create()` to create a > > * new, empty session and receives a file descriptor for it. > > * > > - * 2. Serialization: When the `reboot(LINUX_REBOOT_CMD_KEXEC)` syscall is > > - * made, `luo_session_serialize()` is called. It iterates through all > > - * active sessions and writes their metadata into a memory area preserved > > - * by KHO. > > + * 2. Freeze / Serialization: Sessions may be frozen early from userspace > > + * via `LIVEUPDATE_SESSION_FREEZE`. When the `reboot(LINUX_REBOOT_CMD_KEXEC)` > > + * syscall is made, `luo_session_serialize()` is called to freeze any > > + * remaining active sessions and write all session metadata into a memory > > + * area preserved by KHO. > > * > > * 3. Deserialization (in new kernel): After kexec, `luo_session_deserialize()` > > * runs, reading the serialized data and creating a list of `struct > > @@ -222,6 +223,10 @@ static void luo_session_unfreeze_one(struct luo_session *session, > > struct luo_session_ser *ser) > > { > > guard(mutex)(&session->mutex); > > + if (session->frozen) { > > + memset(&ser->file_set_ser, 0, sizeof(ser->file_set_ser)); > > + return; > > + } > > luo_file_unfreeze(&session->file_set, &ser->file_set_ser); > > } > > > > @@ -229,6 +234,12 @@ static int luo_session_freeze_one(struct luo_session *session, > > struct luo_session_ser *ser) > > { > > guard(mutex)(&session->mutex); > > + if (session->frozen) { > > + ser->file_set_ser.count = session->file_set.count; > > + ser->file_set_ser.files = > > + kho_block_set_head_pa(&session->file_set.block_set); > > + return 0; > > + } > > This is getting messy. If early freeze, then you set the file_set_ser > here, if late freeze, you set it in luo_file_freeze(). > > I think it is better to just set it here, and drop the code from > luo_file_freeze() entirely. > > Same for unfreeze. > > Maybe also rename the function to make it clear it only runs for > serialization? The core of serialization is done by luo_file_freeze(). Agreed, that is cleaner. In v2 I dropped struct luo_file_set_ser * from luo_file_freeze() and luo_file_unfreeze() completely, and renamed the session helpers to luo_session_serialize_one() / luo_session_unserialize_one() so all of struct luo_session_ser is populated and cleared there. Pasha > > > return luo_file_freeze(&session->file_set, &ser->file_set_ser); > > } > > > > @@ -249,8 +260,11 @@ static int luo_session_release(struct inode *inodep, struct file *filep) > > } > > sh = &luo_session_global.incoming; > > } else { > > - scoped_guard(mutex, &session->mutex) > > + scoped_guard(mutex, &session->mutex) { > > + if (session->frozen) > > + luo_file_unfreeze(&session->file_set, NULL); > > luo_file_unpreserve_files(&session->file_set); > > + } > > sh = &luo_session_global.outgoing; > > } > > > > @@ -267,6 +281,9 @@ static int luo_session_preserve_fd(struct luo_session *session, > > int err; > > > > guard(mutex)(&session->mutex); > > + if (session->frozen) > > + return -EBUSY; > > + > > err = luo_preserve_file(&session->file_set, argp->token, argp->fd); > > if (err) > > return err; > > @@ -340,11 +357,34 @@ static int luo_session_get_name(struct luo_session *session, > > return luo_ucmd_respond(ucmd, sizeof(*argp)); > > } > > > > +static int luo_session_freeze(struct luo_session *session, > > + struct luo_ucmd *ucmd) > > +{ > > + struct liveupdate_session_freeze *argp = ucmd->cmd; > > + int err; > > + > > + if (argp->reserved) > > + return -EINVAL; > > + > > + guard(mutex)(&session->mutex); > > + if (session->frozen) > > + return -EBUSY; > > + > > + err = luo_file_freeze(&session->file_set, NULL); > > + if (err) > > + return err; > > + > > + session->frozen = true; > > + > > + return luo_ucmd_respond(ucmd, sizeof(*argp)); > > +} > > + > [...] > > -- > Regards, > Pratyush Yadav