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

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Message-ID: <mafs03496w0kk.fsf@kernel.org>
Date: Mon, 01 Sep 2025 19:21:31 +0200
From: Pratyush Yadav <pratyush@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Mike Rapoport <rppt@...nel.org>,  Jason Gunthorpe <jgg@...dia.com>,
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

Hi Pasha,

On Mon, Sep 01 2025, Pasha Tatashin wrote:

> On Mon, Sep 1, 2025 at 4:23 PM Mike Rapoport <rppt@...nel.org> 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
>
> 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
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/.

As for the store_object/restore_object proposal: see an alternate idea
at [1].

[1] https://lore.kernel.org/lkml/mafs0h5xmw12a.fsf@kernel.org/

-- 
Regards,
Pratyush Yadav