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

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Message-ID: <CA+CK2bCbjmRKtVVAok7GH8xvh8JWrga5Oj-iK-p=1M79AqvhRA@mail.gmail.com>
Date: Thu, 14 Aug 2025 15:05:04 +0000
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Jason Gunthorpe <jgg@...dia.com>
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Subject: 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 <jgg@...dia.com> wrote:
>
> 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?

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