Re: [PATCH v3 07/30] kho: add interfaces to unpreserve folios and physical memory ranges

On Thu, Aug 14, 2025 at 1:22 PM Jason Gunthorpe [off-list ref] wrote:

On Thu, Aug 07, 2025 at 01:44:13AM +0000, Pasha Tatashin wrote:

quoted

+int kho_unpreserve_phys(phys_addr_t phys, size_t size)
+{

Why are we adding phys apis? Didn't we talk about this before and
agree not to expose these?

It is already there, this patch simply completes a lacking unpreserve part.

We can talk about removing it in the future, but the phys interface
provides a benefit of not having to preserve  power of two in length
objects.

The places using it are goofy:

+static int luo_fdt_setup(void)
+{
+       fdt_out = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
+                                          get_order(LUO_FDT_SIZE));

+       ret = kho_preserve_phys(__pa(fdt_out), LUO_FDT_SIZE);

+       WARN_ON_ONCE(kho_unpreserve_phys(__pa(fdt_out), LUO_FDT_SIZE));

It literally allocated a page and then for some reason switches to
phys with an open coded __pa??

This is ugly, if you want a helper to match __get_free_pages() then
make one that works on void * directly. You can get the order of the
void * directly from the struct page IIRC when using GFP_COMP.

I will make this changes.

Which is perhaps another comment, if this __get_free_pages() is going
to be a common pattern (and I guess it will be) then the API should be
streamlined alot more:

 void *kho_alloc_preserved_memory(gfp, size);
 void kho_free_preserved_memory(void *);

Hm, not all GFP flags are compatible with KHO preserve, but we could
add this or similar API, but first let's make KHO completely
stateless: remove, finalize and abort parts from it.

Which can wrapper the get_free_pages and the preserve logic and gives
a nice path to possibly someday supporting non-PAGE_SIZE allocations.

Jason