Re: [PATCH RFC 1/4] mm/swap: Add swp_offset_pfn() to fetch PFN from swap entry

[Huang, Ying]

Peter Xu <peterx@xxxxxxxxxx> writes:

> We've got a bunch of special swap entries that stores PFN inside the swap
> offset fields.  To fetch the PFN, normally the user just calls swp_offset()
> assuming that'll be the PFN.
>
> Add a helper swp_offset_pfn() to fetch the PFN instead, fetching only the
> max possible length of a PFN on the host, meanwhile doing proper check with
> MAX_PHYSMEM_BITS to make sure the swap offsets can actually store the PFNs
> properly always using the BUILD_BUG_ON() in is_pfn_swap_entry().
>
> One reason to do so is we never tried to sanitize whether swap offset can
> really fit for storing PFN.  At the meantime, this patch also prepares us
> with the future possibility to store more information inside the swp offset
> field, so assuming "swp_offset(entry)" to be the PFN will not stand any
> more very soon.
>
> Replace many of the swp_offset() callers to use swp_offset_pfn() where
> proper.  Note that many of the existing users are not candidates for the
> replacement, e.g.:
>
>   (1) When the swap entry is not a pfn swap entry at all, or,
>   (2) when we wanna keep the whole swp_offset but only change the swp type.
>
> For the latter, it can happen when fork() triggered on a write-migration
> swap entry pte, we may want to only change the migration type from
> write->read but keep the rest, so it's not "fetching PFN" but "changing
> swap type only".  They're left aside so that when there're more information
> within the swp offset they'll be carried over naturally in those cases.
>
> Since at it, dropping hwpoison_entry_to_pfn() because that's exactly what
> the new swp_offset_pfn() is about.
>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  arch/arm64/mm/hugetlbpage.c |  2 +-
>  include/linux/swapops.h     | 28 ++++++++++++++++++++++------
>  mm/hmm.c                    |  2 +-
>  mm/memory-failure.c         |  2 +-
>  mm/page_vma_mapped.c        |  6 +++---
>  5 files changed, 28 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index 7430060cb0d6..f897d40821dd 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -242,7 +242,7 @@ static inline struct folio *hugetlb_swap_entry_to_folio(swp_entry_t entry)
>  {
>  	VM_BUG_ON(!is_migration_entry(entry) && !is_hwpoison_entry(entry));
>  
> -	return page_folio(pfn_to_page(swp_offset(entry)));
> +	return page_folio(pfn_to_page(swp_offset_pfn(entry)));
>  }
>  
>  void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index a3d435bf9f97..5378f77860fb 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -23,6 +23,14 @@
>  #define SWP_TYPE_SHIFT	(BITS_PER_XA_VALUE - MAX_SWAPFILES_SHIFT)
>  #define SWP_OFFSET_MASK	((1UL << SWP_TYPE_SHIFT) - 1)
>  
> +/*
> + * Definitions only for PFN swap entries (see is_pfn_swap_entry()).  To
> + * store PFN, we only need SWP_PFN_BITS bits.  Each of the pfn swap entries
> + * can use the extra bits to store other information besides PFN.
> + */
> +#define SWP_PFN_BITS			(MAX_PHYSMEM_BITS - PAGE_SHIFT)
> +#define SWP_PFN_MASK			((1UL << SWP_PFN_BITS) - 1)
> +
>  /* Clear all flags but only keep swp_entry_t related information */
>  static inline pte_t pte_swp_clear_flags(pte_t pte)
>  {
> @@ -64,6 +72,16 @@ static inline pgoff_t swp_offset(swp_entry_t entry)
>  	return entry.val & SWP_OFFSET_MASK;
>  }
>  
> +/*
> + * This should only be called upon a pfn swap entry to get the PFN stored
> + * in the swap entry.  Please refers to is_pfn_swap_entry() for definition
> + * of pfn swap entry.
> + */
> +static inline unsigned long swp_offset_pfn(swp_entry_t entry)
> +{

Is it good to call is_pfn_swap_entry() here for debug that can be
eliminated in the production kernel?

> +	return swp_offset(entry) & SWP_PFN_MASK;
> +}
> +
>  /* check whether a pte points to a swap entry */
>  static inline int is_swap_pte(pte_t pte)
>  {
> @@ -369,7 +387,7 @@ static inline int pte_none_mostly(pte_t pte)
>  
>  static inline struct page *pfn_swap_entry_to_page(swp_entry_t entry)
>  {
> -	struct page *p = pfn_to_page(swp_offset(entry));
> +	struct page *p = pfn_to_page(swp_offset_pfn(entry));
>  
>  	/*
>  	 * Any use of migration entries may only occur while the
> @@ -387,6 +405,9 @@ static inline struct page *pfn_swap_entry_to_page(swp_entry_t entry)
>   */
>  static inline bool is_pfn_swap_entry(swp_entry_t entry)
>  {
> +	/* Make sure the swp offset can always store the needed fields */
> +	BUILD_BUG_ON(SWP_TYPE_SHIFT < SWP_PFN_BITS);

	BUILD_BUG_ON(SWP_TYPE_SHIFT <= SWP_PFN_BITS);

?

Best Regards,
Huang, Ying

> +
>  	return is_migration_entry(entry) || is_device_private_entry(entry) ||
>  	       is_device_exclusive_entry(entry);
>  }
> @@ -475,11 +496,6 @@ static inline int is_hwpoison_entry(swp_entry_t entry)
>  	return swp_type(entry) == SWP_HWPOISON;
>  }
>  
> -static inline unsigned long hwpoison_entry_to_pfn(swp_entry_t entry)
> -{
> -	return swp_offset(entry);
> -}
> -
>  static inline void num_poisoned_pages_inc(void)
>  {
>  	atomic_long_inc(&num_poisoned_pages);
> diff --git a/mm/hmm.c b/mm/hmm.c
> index f2aa63b94d9b..3850fb625dda 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -253,7 +253,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>  			cpu_flags = HMM_PFN_VALID;
>  			if (is_writable_device_private_entry(entry))
>  				cpu_flags |= HMM_PFN_WRITE;
> -			*hmm_pfn = swp_offset(entry) | cpu_flags;
> +			*hmm_pfn = swp_offset_pfn(entry) | cpu_flags;
>  			return 0;
>  		}
>  
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index cc6fc9be8d22..e451219124dd 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -632,7 +632,7 @@ static int check_hwpoisoned_entry(pte_t pte, unsigned long addr, short shift,
>  		swp_entry_t swp = pte_to_swp_entry(pte);
>  
>  		if (is_hwpoison_entry(swp))
> -			pfn = hwpoison_entry_to_pfn(swp);
> +			pfn = swp_offset_pfn(swp);
>  	}
>  
>  	if (!pfn || pfn != poisoned_pfn)
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 8e9e574d535a..93e13fc17d3c 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -86,7 +86,7 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
>  		    !is_device_exclusive_entry(entry))
>  			return false;
>  
> -		pfn = swp_offset(entry);
> +		pfn = swp_offset_pfn(entry);
>  	} else if (is_swap_pte(*pvmw->pte)) {
>  		swp_entry_t entry;
>  
> @@ -96,7 +96,7 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
>  		    !is_device_exclusive_entry(entry))
>  			return false;
>  
> -		pfn = swp_offset(entry);
> +		pfn = swp_offset_pfn(entry);
>  	} else {
>  		if (!pte_present(*pvmw->pte))
>  			return false;
> @@ -221,7 +221,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  					return not_found(pvmw);
>  				entry = pmd_to_swp_entry(pmde);
>  				if (!is_migration_entry(entry) ||
> -				    !check_pmd(swp_offset(entry), pvmw))
> +				    !check_pmd(swp_offset_pfn(entry), pvmw))
>  					return not_found(pvmw);
>  				return true;
>  			}