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

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Message-ID: <mafs0ldmyw1hp.fsf@kernel.org>
Date: Mon, 01 Sep 2025 19:01:38 +0200
From: Pratyush Yadav <pratyush@...nel.org>
To: Mike Rapoport <rppt@...nel.org>
Cc: Jason Gunthorpe <jgg@...dia.com>,  Pasha Tatashin
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

Hi Mike,

On Mon, Sep 01 2025, Mike Rapoport wrote:

> On Tue, Aug 26, 2025 at 01:20:19PM -0300, Jason Gunthorpe wrote:
>> On Thu, Aug 07, 2025 at 01:44:35AM +0000, Pasha Tatashin wrote:
>> 
>> > +	/*
>> > +	 * Most of the space should be taken by preserved folios. So take its
>> > +	 * size, plus a page for other properties.
>> > +	 */
>> > +	fdt = memfd_luo_create_fdt(PAGE_ALIGN(preserved_size) + PAGE_SIZE);
>> > +	if (!fdt) {
>> > +		err = -ENOMEM;
>> > +		goto err_unpin;
>> > +	}
>> 
>> This doesn't seem to have any versioning scheme, it really should..
>> 
>> > +	err = fdt_property_placeholder(fdt, "folios", preserved_size,
>> > +				       (void **)&preserved_folios);
>> > +	if (err) {
>> > +		pr_err("Failed to reserve folios property in FDT: %s\n",
>> > +		       fdt_strerror(err));
>> > +		err = -ENOMEM;
>> > +		goto err_free_fdt;
>> > +	}
>> 
>> Yuk.
>> 
>> This really wants some luo helper
>> 
>> 'luo alloc array'
>> 'luo restore array'
>> 'luo free array'
>
> We can just add kho_{preserve,restore}_vmalloc(). I've drafted it here:
> https://git.kernel.org/pub/scm/linux/kernel/git/rppt/linux.git/log/?h=kho/vmalloc/v1
>
> Will wait for kbuild and then send proper patches.

I have been working on something similar, but in a more generic way.

I have implemented a sparse KHO-preservable array (called kho_array)
with xarray like properties. It can take in 4-byte aligned pointers and
supports saving non-pointer values similar to xa_mk_value(). For now it
doesn't support multi-index entries, but if needed the data format can
be extended to support it as well.

The structure is very similar to what you have implemented. It uses a
linked list of pages with some metadata at the head of each page.

I have used it for memfd preservation, and I think it is quite
versatile. For example, your kho_preserve_vmalloc() can be very easily
built on top of this kho_array by simply saving each physical page
address at consecutive indices in the array.

The code is still WIP and currently a bit hacky, but I will clean it up
in a couple days and I think it should be ready for posting. You can
find the current version at [0][1]. Would be good to hear your thoughts,
and if you agree with the approach, I can also port
kho_preserve_vmalloc() to work on top of kho_array as well.

[0] https://git.kernel.org/pub/scm/linux/kernel/git/pratyush/linux.git/commit/?h=kho-array&id=cf4c04c1e9ac854e3297018ad6dada17c54a59af
[1] https://git.kernel.org/pub/scm/linux/kernel/git/pratyush/linux.git/commit/?h=kho-array&id=5eb0d7316274a9c87acaeedd86941979fc4baf96

-- 
Regards,
Pratyush Yadav