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

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Message-ID: <20250814132233.GB802098@nvidia.com>
Date: Thu, 14 Aug 2025 10:22:33 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
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Subject: Re: [PATCH v3 07/30] kho: add interfaces to unpreserve folios and
 physical memory ranges

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

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.

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 *);

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