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

On Mon, Sep 1, 2025 at 4:23 PM Mike Rapoport [off-list ref] wrote:

On Tue, Aug 26, 2025 at 01:20:19PM -0300, Jason Gunthorpe wrote:

quoted

On Thu, Aug 07, 2025 at 01:44:35AM +0000, Pasha Tatashin wrote:

quoted

+   /*
+    * 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..

quoted

+   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.

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.

Will wait for kbuild and then send proper patches.


--
Sincerely yours,
Mike.