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

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Message-ID: <CA+CK2bAb6s=gUTCNjMrOqptZ3a_nj3teuVSZs86AvVymvaURQA@mail.gmail.com>
Date: Mon, 1 Sep 2025 19:02:46 +0000
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Mike Rapoport <rppt@...nel.org>, Jason Gunthorpe <jgg@...dia.com>, jasonmiu@...gle.com, 
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

> >> > 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
> >
> > The patch looks okay to me, but it doesn't support holes in vmap
> > areas. While that is likely acceptable for vmalloc, it could be a
> > problem if we want to preserve memfd with holes and using vmap
> > preservation as a method, which would require a different approach.
> > Still, this would help with preserving memfd.
>
> I agree. I think we should do it the other way round. Build a sparse
> array first, and then use that to build vmap preservation. Our emails

Yes, sparse array support would help both: vmalloc and memfd preservation.

> seem to have crossed, but see my reply to Mike [0] that describes my
> idea a bit more, along with WIP code.
>
> [0] https://lore.kernel.org/lkml/mafs0ldmyw1hp.fsf@kernel.org/
>
> >
> > However, I wonder if we should add a separate preservation library on
> > top of the kho and not as part of kho (or at least keep them in a
> > separate file from core logic). This would allow us to preserve more
> > advanced data structures such as this and define preservation version
> > control, similar to Jason's store_object/restore_object proposal.
>
> This is how I have done it in my code: created a separate file called
> kho_array.c. If we have enough such data structures, we can probably
> move it under kernel/liveupdate/lib/.

Yes, let's place it under kernel/liveupdate/lib/. We will add more
preservation types over time.

> As for the store_object/restore_object proposal: see an alternate idea
> at [1].
>
> [1] https://lore.kernel.org/lkml/mafs0h5xmw12a.fsf@kernel.org/

What you are proposing makes sense. We can update the LUO API to be
responsible for passing the compatible string outside of the data
payload. However, I think we first need to settle on the actual API
for storing and restoring a versioned blob of data and place that code
into kernel/liveupdate/lib/. Depending on which API we choose, we can
then modify the LUO to work accordingly.

>
> --
> Regards,
> Pratyush Yadav