Re: [RFC v2 03/16] kho: add kho_unpreserve_folio/phys

[Pratyush Yadav <pratyush@kernel.org>]

On Thu, May 15 2025, Pasha Tatashin wrote:

> From: Changyuan Lyu <changyuanl@google.com>
>
> Allow users of KHO to cancel the previous preservation by adding the
> necessary interfaces to unpreserve folio.
>
> Signed-off-by: Changyuan Lyu <changyuanl@google.com>
> Co-developed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
>  include/linux/kexec_handover.h | 12 +++++
>  kernel/kexec_handover.c        | 84 ++++++++++++++++++++++++++++------
>  2 files changed, 83 insertions(+), 13 deletions(-)
>
[...]
> diff --git a/kernel/kexec_handover.c b/kernel/kexec_handover.c
> index 8ff561e36a87..eb305e7e6129 100644
> --- a/kernel/kexec_handover.c
> +++ b/kernel/kexec_handover.c
> @@ -101,26 +101,33 @@ static void *xa_load_or_alloc(struct xarray *xa, unsigned long index, size_t sz)
>  	return elm;
>  }
>  
> -static void __kho_unpreserve(struct kho_mem_track *track, unsigned long pfn,
> -			     unsigned long end_pfn)
> +static void __kho_unpreserve_order(struct kho_mem_track *track, unsigned long pfn,
> +				   unsigned int order)
>  {
>  	struct kho_mem_phys_bits *bits;
>  	struct kho_mem_phys *physxa;
> +	const unsigned long pfn_high = pfn >> order;
>  
> -	while (pfn < end_pfn) {
> -		const unsigned int order =
> -			min(count_trailing_zeros(pfn), ilog2(end_pfn - pfn));
> -		const unsigned long pfn_high = pfn >> order;
> +	physxa = xa_load(&track->orders, order);
> +	if (!physxa)
> +		return;
>  
> -		physxa = xa_load(&track->orders, order);
> -		if (!physxa)
> -			continue;
> +	bits = xa_load(&physxa->phys_bits, pfn_high / PRESERVE_BITS);
> +	if (!bits)
> +		return;
>  
> -		bits = xa_load(&physxa->phys_bits, pfn_high / PRESERVE_BITS);
> -		if (!bits)
> -			continue;
> +	clear_bit(pfn_high % PRESERVE_BITS, bits->preserve);
> +}
>  
> -		clear_bit(pfn_high % PRESERVE_BITS, bits->preserve);
> +static void __kho_unpreserve(struct kho_mem_track *track, unsigned long pfn,
> +			     unsigned long end_pfn)
> +{
> +	unsigned int order;
> +
> +	while (pfn < end_pfn) {
> +		order = min(count_trailing_zeros(pfn), ilog2(end_pfn - pfn));

This is fragile. If the preserve call spans say 4 PFNs, then it gets
preserved as a order 2 allocation, but if the PFNs are unpreserved
one-by-one, __kho_unpreserve_order() will unpreserve from the order 0
xarray, which will end up doing nothing, leaking those pages.

It should either look through all orders to find the PFN, or at least
have a requirement in the API that the same phys and size combination as
the preserve call must be given to unpreserve.

> +
> +		__kho_unpreserve_order(track, pfn, order);
>  
>  		pfn += 1 << order;
>  	}
> @@ -607,6 +614,29 @@ int kho_preserve_folio(struct folio *folio)
>  }
>  EXPORT_SYMBOL_GPL(kho_preserve_folio);
>  
> +/**
> + * kho_unpreserve_folio - unpreserve a folio.
> + * @folio: folio to unpreserve.
> + *
> + * Instructs KHO to unpreserve a folio that was preserved by
> + * kho_preserve_folio() before.
> + *
> + * Return: 0 on success, error code on failure
> + */
> +int kho_unpreserve_folio(struct folio *folio)
> +{
> +	const unsigned long pfn = folio_pfn(folio);
> +	const unsigned int order = folio_order(folio);
> +	struct kho_mem_track *track = &kho_out.ser.track;
> +
> +	if (kho_out.finalized)
> +		return -EBUSY;
> +
> +	__kho_unpreserve_order(track, pfn, order);
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(kho_unpreserve_folio);
> +
>  /**
>   * kho_preserve_phys - preserve a physically contiguous range across kexec.
>   * @phys: physical address of the range.
> @@ -652,6 +682,34 @@ int kho_preserve_phys(phys_addr_t phys, size_t size)
>  }
>  EXPORT_SYMBOL_GPL(kho_preserve_phys);
>  
> +/**
> + * kho_unpreserve_phys - unpreserve a physically contiguous range across kexec.
> + * @phys: physical address of the range.
> + * @size: size of the range.
> + *
> + * Instructs KHO to unpreserve the memory range from @phys to @phys + @size
> + * across kexec.
> + *
> + * Return: 0 on success, error code on failure
> + */
> +int kho_unpreserve_phys(phys_addr_t phys, size_t size)
> +{
> +	struct kho_mem_track *track = &kho_out.ser.track;
> +	unsigned long pfn = PHYS_PFN(phys);
> +	unsigned long end_pfn = PHYS_PFN(phys + size);
> +
> +	if (kho_out.finalized)
> +		return -EBUSY;
> +
> +	if (!PAGE_ALIGNED(phys) || !PAGE_ALIGNED(size))
> +		return -EINVAL;
> +
> +	__kho_unpreserve(track, pfn, end_pfn);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(kho_unpreserve_phys);
> +
>  int __kho_abort(void)
>  {
>  	int err;

-- 
Regards,
Pratyush Yadav