Re: [PATCH v2 12/13] mm/gup: move gup_must_unshare() to mm/internal.h

[John Hubbard <jhubbard@nvidia.com>]

On 1/24/23 12:34, Jason Gunthorpe wrote:
> This function is only used in gup.c and closely related. It touches
> FOLL_PIN so it must be moved before the next patch.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>   include/linux/mm.h | 65 ----------------------------------------------
>   mm/internal.h      | 65 ++++++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 65 insertions(+), 65 deletions(-)

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA

> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a47a6e8a9c78be..e0bacf9f2c5ebe 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3087,71 +3087,6 @@ static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags)
>   	return 0;
>   }
>   
> -/*
> - * Indicates for which pages that are write-protected in the page table,
> - * whether GUP has to trigger unsharing via FAULT_FLAG_UNSHARE such that the
> - * GUP pin will remain consistent with the pages mapped into the page tables
> - * of the MM.
> - *
> - * Temporary unmapping of PageAnonExclusive() pages or clearing of
> - * PageAnonExclusive() has to protect against concurrent GUP:
> - * * Ordinary GUP: Using the PT lock
> - * * GUP-fast and fork(): mm->write_protect_seq
> - * * GUP-fast and KSM or temporary unmapping (swap, migration): see
> - *    page_try_share_anon_rmap()
> - *
> - * Must be called with the (sub)page that's actually referenced via the
> - * page table entry, which might not necessarily be the head page for a
> - * PTE-mapped THP.
> - *
> - * If the vma is NULL, we're coming from the GUP-fast path and might have
> - * to fallback to the slow path just to lookup the vma.
> - */
> -static inline bool gup_must_unshare(struct vm_area_struct *vma,
> -				    unsigned int flags, struct page *page)
> -{
> -	/*
> -	 * FOLL_WRITE is implicitly handled correctly as the page table entry
> -	 * has to be writable -- and if it references (part of) an anonymous
> -	 * folio, that part is required to be marked exclusive.
> -	 */
> -	if ((flags & (FOLL_WRITE | FOLL_PIN)) != FOLL_PIN)
> -		return false;
> -	/*
> -	 * Note: PageAnon(page) is stable until the page is actually getting
> -	 * freed.
> -	 */
> -	if (!PageAnon(page)) {
> -		/*
> -		 * We only care about R/O long-term pining: R/O short-term
> -		 * pinning does not have the semantics to observe successive
> -		 * changes through the process page tables.
> -		 */
> -		if (!(flags & FOLL_LONGTERM))
> -			return false;
> -
> -		/* We really need the vma ... */
> -		if (!vma)
> -			return true;
> -
> -		/*
> -		 * ... because we only care about writable private ("COW")
> -		 * mappings where we have to break COW early.
> -		 */
> -		return is_cow_mapping(vma->vm_flags);
> -	}
> -
> -	/* Paired with a memory barrier in page_try_share_anon_rmap(). */
> -	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
> -		smp_rmb();
> -
> -	/*
> -	 * Note that PageKsm() pages cannot be exclusive, and consequently,
> -	 * cannot get pinned.
> -	 */
> -	return !PageAnonExclusive(page);
> -}
> -
>   /*
>    * Indicates whether GUP can follow a PROT_NONE mapped page, or whether
>    * a (NUMA hinting) fault is required.
> diff --git a/mm/internal.h b/mm/internal.h
> index 0f035bcaf133f5..5c1310b98db64d 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -854,6 +854,71 @@ int migrate_device_coherent_page(struct page *page);
>   struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags);
>   int __must_check try_grab_page(struct page *page, unsigned int flags);
>   
> +/*
> + * Indicates for which pages that are write-protected in the page table,
> + * whether GUP has to trigger unsharing via FAULT_FLAG_UNSHARE such that the
> + * GUP pin will remain consistent with the pages mapped into the page tables
> + * of the MM.
> + *
> + * Temporary unmapping of PageAnonExclusive() pages or clearing of
> + * PageAnonExclusive() has to protect against concurrent GUP:
> + * * Ordinary GUP: Using the PT lock
> + * * GUP-fast and fork(): mm->write_protect_seq
> + * * GUP-fast and KSM or temporary unmapping (swap, migration): see
> + *    page_try_share_anon_rmap()
> + *
> + * Must be called with the (sub)page that's actually referenced via the
> + * page table entry, which might not necessarily be the head page for a
> + * PTE-mapped THP.
> + *
> + * If the vma is NULL, we're coming from the GUP-fast path and might have
> + * to fallback to the slow path just to lookup the vma.
> + */
> +static inline bool gup_must_unshare(struct vm_area_struct *vma,
> +				    unsigned int flags, struct page *page)
> +{
> +	/*
> +	 * FOLL_WRITE is implicitly handled correctly as the page table entry
> +	 * has to be writable -- and if it references (part of) an anonymous
> +	 * folio, that part is required to be marked exclusive.
> +	 */
> +	if ((flags & (FOLL_WRITE | FOLL_PIN)) != FOLL_PIN)
> +		return false;
> +	/*
> +	 * Note: PageAnon(page) is stable until the page is actually getting
> +	 * freed.
> +	 */
> +	if (!PageAnon(page)) {
> +		/*
> +		 * We only care about R/O long-term pining: R/O short-term
> +		 * pinning does not have the semantics to observe successive
> +		 * changes through the process page tables.
> +		 */
> +		if (!(flags & FOLL_LONGTERM))
> +			return false;
> +
> +		/* We really need the vma ... */
> +		if (!vma)
> +			return true;
> +
> +		/*
> +		 * ... because we only care about writable private ("COW")
> +		 * mappings where we have to break COW early.
> +		 */
> +		return is_cow_mapping(vma->vm_flags);
> +	}
> +
> +	/* Paired with a memory barrier in page_try_share_anon_rmap(). */
> +	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
> +		smp_rmb();
> +
> +	/*
> +	 * Note that PageKsm() pages cannot be exclusive, and consequently,
> +	 * cannot get pinned.
> +	 */
> +	return !PageAnonExclusive(page);
> +}
> +
>   extern bool mirrored_kernelcore;
>   
>   static inline bool vma_soft_dirty_enabled(struct vm_area_struct *vma)