linux-kernel - Re: [PATCH v3 29/30] luo: allow preserving memfd

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Message-ID: <20250902134846.GN186519@nvidia.com>
Date: Tue, 2 Sep 2025 10:48:46 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Pratyush Yadav <pratyush@...nel.org>
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

On Mon, Sep 01, 2025 at 07:10:53PM +0200, Pratyush Yadav wrote:
> Building kvalloc on top of this becomes trivial.
> 
> [0] https://git.kernel.org/pub/scm/linux/kernel/git/pratyush/linux.git/commit/?h=kho-array&id=cf4c04c1e9ac854e3297018ad6dada17c54a59af

This isn't really an array, it is a non-seekable serialization of
key/values with some optimization for consecutive keys. IMHO it is
most useful if you don't know the size of the thing you want to
serialize in advance since it has a nice dynamic append.

But if you do know the size, I think it makes more sense just to do a
preserving vmalloc and write out a linear array..

So, it could be useful, but I wouldn't use it for memfd, the vmalloc
approach is better and we shouldn't optimize for sparsness which
should never happen.

> > The versioning should be first class, not hidden away as some emergent
> > property of registering multiple serializers or something like that.
> 
> That makes sense. How about some simple changes to the LUO interfaces to
> make the version more prominent:
> 
> 	int (*prepare)(struct liveupdate_file_handler *handler,
> 		       struct file *file, u64 *data, char **compatible);

Yeah, something more integrated with the ops is better.

You could list the supported versions in the ops itself

  const char **supported_deserialize_versions;

And let the luo framework find the right versions.

But for prepare I would expect an inbetween object:

	int (*prepare)(struct liveupdate_file_handler *handler,
	    	       struct luo_object *obj, struct file *file);

And then you'd do function calls on 'obj' to store 'data' per version.

Jason