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

[Mike Rapoport <rppt@kernel.org>]

Hi Pratyush,

On Mon, Sep 01, 2025 at 07:01:38PM +0200, Pratyush Yadav wrote:
> 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.

I've started to work on something similar to your kho_array for memfd case
and then I thought that since we know the size of the array we can simply
vmalloc it and preserve vmalloc, and that lead me to implementing
preservation of vmalloc :)

I like the idea to have kho_array for cases when we don't know the amount
of data to preserve in advance, but for memfd as it's currently
implemented I think that allocating and preserving vmalloc is simpler.

As for porting kho_preserve_vmalloc() to kho_array, I also feel that it
would just make kho_preserve_vmalloc() more complex and I'd rather simplify
it even more, e.g. with preallocating all the pages that preserve indices
in advance.
 
> 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

-- 
Sincerely yours,
Mike.