linux-kernel - Re: [RFC v2 08/16] luo: luo_files: add infrastructure for FDs

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Message-ID: <mafs07c1qtnah.fsf@kernel.org>
Date: Thu, 05 Jun 2025 13:56:06 +0200
From: Pratyush Yadav <pratyush@...nel.org>
To: Mike Rapoport <rppt@...nel.org>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>,  pratyush@...nel.org,
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Subject: Re: [RFC v2 08/16] luo: luo_files: add infrastructure for FDs

On Mon, May 26 2025, Mike Rapoport wrote:

> On Thu, May 15, 2025 at 06:23:12PM +0000, Pasha Tatashin wrote:
>> Introduce the framework within LUO to support preserving specific types
>> of file descriptors across a live update transition. This allows
>> stateful FDs (like memfds or vfio FDs used by VMs) to be recreated in
>> the new kernel.
>> 
>> Note: The core logic for iterating through the luo_files_list and
>> invoking the handler callbacks (prepare, freeze, cancel, finish)
>> within luo_do_files_*_calls, as well as managing the u64 data
>> persistence via the FDT for individual files, is currently implemented
>> as stubs in this patch. This patch sets up the registration, FDT layout,
>> and retrieval framework.
>> 
>> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
>> ---
[...]
>> diff --git a/drivers/misc/liveupdate/luo_core.c b/drivers/misc/liveupdate/luo_core.c
>> index 417e7f6bf36c..ab1d76221fe2 100644
>> --- a/drivers/misc/liveupdate/luo_core.c
>> +++ b/drivers/misc/liveupdate/luo_core.c
>> @@ -110,6 +110,10 @@ static int luo_fdt_setup(struct kho_serialization *ser)
>>  	if (ret)
>>  		goto exit_free;
>>  
>> +	ret = luo_files_fdt_setup(fdt_out);
>> +	if (ret)
>> +		goto exit_free;
>> +
>>  	ret = luo_subsystems_fdt_setup(fdt_out);
>>  	if (ret)
>>  		goto exit_free;
>
> The duplication of files and subsystems does not look nice here and below.
> Can't we make files to be a subsystem?

+1

It would also give subsystems a user.

[...]
>> diff --git a/drivers/misc/liveupdate/luo_files.c b/drivers/misc/liveupdate/luo_files.c
>> new file mode 100644
>> index 000000000000..953fc40db3d7
>> --- /dev/null
>> +++ b/drivers/misc/liveupdate/luo_files.c
>> @@ -0,0 +1,563 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +/*
>> + * Copyright (c) 2025, Google LLC.
>> + * Pasha Tatashin <pasha.tatashin@...een.com>
>> + */
>> +
>> +/**
>> + * DOC: LUO file descriptors
>> + *
>> + * LUO provides the infrastructure necessary to preserve
>> + * specific types of stateful file descriptors across a kernel live
>> + * update transition. The primary goal is to allow workloads, such as virtual
>> + * machines using vfio, memfd, or iommufd to retain access to their essential
>> + * resources without interruption after the underlying kernel is  updated.
>> + *
>> + * The framework operates based on handler registration and instance tracking:
>> + *
>> + * 1. Handler Registration: Kernel modules responsible for specific file
>> + * types (e.g., memfd, vfio) register a &struct liveupdate_filesystem
>> + * handler. This handler contains callbacks (&liveupdate_filesystem.prepare,
>> + * &liveupdate_filesystem.freeze, &liveupdate_filesystem.finish, etc.)
>> + * and a unique 'compatible' string identifying the file type.
>> + * Registration occurs via liveupdate_register_filesystem().
>
> I wouldn't use filesystem here, as the obvious users are not really
> filesystems. Maybe liveupdate_register_file_ops?

+1

[...]

-- 
Regards,
Pratyush Yadav