Re: [PATCH v2 15/28] gup: Add try_get_folio() and try_grab_folio()

[John Hubbard <jhubbard@nvidia.com>]

On 1/9/22 20:23, Matthew Wilcox (Oracle) wrote:
> Convert try_get_compound_head() into try_get_folio() and convert
> try_grab_compound_head() into try_grab_folio().  Also convert
> hpage_pincount_add() to folio_pincount_add().  Add a temporary
> try_grab_compound_head() wrapper around try_grab_folio() to
> let us convert callers individually.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>   mm/gup.c      | 104 +++++++++++++++++++++++++-------------------------
>   mm/internal.h |   5 +++
>   2 files changed, 56 insertions(+), 53 deletions(-)

Still looks good.

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


thanks,
-- 
John Hubbard
NVIDIA

> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 1282d29357b7..9e581201d679 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -30,21 +30,19 @@ struct follow_page_context {
>   };
>   
>   /*
> - * When pinning a compound page, use an exact count to track it, via
> - * page_pincount_add/_sub().
> + * When pinning a large folio, use an exact count to track it.
>    *
> - * However, be sure to *also* increment the normal page refcount field
> - * at least once, so that the page really is pinned.  That's why the
> - * refcount from the earlier try_get_compound_head() is left intact.
> + * However, be sure to *also* increment the normal folio refcount
> + * field at least once, so that the folio really is pinned.
> + * That's why the refcount from the earlier
> + * try_get_folio() is left intact.
>    */
> -static void page_pincount_add(struct page *page, int refs)
> +static void folio_pincount_add(struct folio *folio, int refs)
>   {
> -	VM_BUG_ON_PAGE(page != compound_head(page), page);
> -
> -	if (PageHead(page))
> -		atomic_add(refs, compound_pincount_ptr(page));
> +	if (folio_test_large(folio))
> +		atomic_add(refs, folio_pincount_ptr(folio));
>   	else
> -		page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
> +		folio_ref_add(folio, refs * (GUP_PIN_COUNTING_BIAS - 1));
>   }
>   
>   static int page_pincount_sub(struct page *page, int refs)
> @@ -76,75 +74,70 @@ static void put_page_refs(struct page *page, int refs)
>   }
>   
>   /*
> - * Return the compound head page with ref appropriately incremented,
> + * Return the folio with ref appropriately incremented,
>    * or NULL if that failed.
>    */
> -static inline struct page *try_get_compound_head(struct page *page, int refs)
> +static inline struct folio *try_get_folio(struct page *page, int refs)
>   {
> -	struct page *head;
> +	struct folio *folio;
>   
>   retry:
> -	head = compound_head(page);
> -
> -	if (WARN_ON_ONCE(page_ref_count(head) < 0))
> +	folio = page_folio(page);
> +	if (WARN_ON_ONCE(folio_ref_count(folio) < 0))
>   		return NULL;
> -	if (unlikely(!page_cache_add_speculative(head, refs)))
> +	if (unlikely(!folio_ref_try_add_rcu(folio, refs)))
>   		return NULL;
>   
>   	/*
> -	 * At this point we have a stable reference to the head page; but it
> -	 * could be that between the compound_head() lookup and the refcount
> -	 * increment, the compound page was split, in which case we'd end up
> -	 * holding a reference on a page that has nothing to do with the page
> +	 * At this point we have a stable reference to the folio; but it
> +	 * could be that between calling page_folio() and the refcount
> +	 * increment, the folio was split, in which case we'd end up
> +	 * holding a reference on a folio that has nothing to do with the page
>   	 * we were given anymore.
> -	 * So now that the head page is stable, recheck that the pages still
> -	 * belong together.
> +	 * So now that the folio is stable, recheck that the page still
> +	 * belongs to this folio.
>   	 */
> -	if (unlikely(compound_head(page) != head)) {
> -		put_page_refs(head, refs);
> +	if (unlikely(page_folio(page) != folio)) {
> +		folio_put_refs(folio, refs);
>   		goto retry;
>   	}
>   
> -	return head;
> +	return folio;
>   }
>   
>   /**
> - * try_grab_compound_head() - attempt to elevate a page's refcount, by a
> - * flags-dependent amount.
> - *
> - * Even though the name includes "compound_head", this function is still
> - * appropriate for callers that have a non-compound @page to get.
> - *
> + * try_grab_folio() - Attempt to get or pin a folio.
>    * @page:  pointer to page to be grabbed
> - * @refs:  the value to (effectively) add to the page's refcount
> + * @refs:  the value to (effectively) add to the folio's refcount
>    * @flags: gup flags: these are the FOLL_* flag values.
>    *
>    * "grab" names in this file mean, "look at flags to decide whether to use
> - * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
> + * FOLL_PIN or FOLL_GET behavior, when incrementing the folio's refcount.
>    *
>    * Either FOLL_PIN or FOLL_GET (or neither) must be set, but not both at the
>    * same time. (That's true throughout the get_user_pages*() and
>    * pin_user_pages*() APIs.) Cases:
>    *
> - *    FOLL_GET: page's refcount will be incremented by @refs.
> + *    FOLL_GET: folio's refcount will be incremented by @refs.
>    *
> - *    FOLL_PIN on compound pages: page's refcount will be incremented by
> - *    @refs, and page[1].compound_pincount will be incremented by @refs.
> + *    FOLL_PIN on large folios: folio's refcount will be incremented by
> + *    @refs, and its compound_pincount will be incremented by @refs.
>    *
> - *    FOLL_PIN on normal pages: page's refcount will be incremented by
> + *    FOLL_PIN on single-page folios: folio's refcount will be incremented by
>    *    @refs * GUP_PIN_COUNTING_BIAS.
>    *
> - * Return: head page (with refcount appropriately incremented) for success, or
> - * NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
> - * considered failure, and furthermore, a likely bug in the caller, so a warning
> - * is also emitted.
> + * Return: The folio containing @page (with refcount appropriately
> + * incremented) for success, or NULL upon failure. If neither FOLL_GET
> + * nor FOLL_PIN was set, that's considered failure, and furthermore,
> + * a likely bug in the caller, so a warning is also emitted.
>    */
> -struct page *try_grab_compound_head(struct page *page,
> -				    int refs, unsigned int flags)
> +struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
>   {
>   	if (flags & FOLL_GET)
> -		return try_get_compound_head(page, refs);
> +		return try_get_folio(page, refs);
>   	else if (flags & FOLL_PIN) {
> +		struct folio *folio;
> +
>   		/*
>   		 * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
>   		 * right zone, so fail and let the caller fall back to the slow
> @@ -158,21 +151,26 @@ struct page *try_grab_compound_head(struct page *page,
>   		 * CAUTION: Don't use compound_head() on the page before this
>   		 * point, the result won't be stable.
>   		 */
> -		page = try_get_compound_head(page, refs);
> -		if (!page)
> +		folio = try_get_folio(page, refs);
> +		if (!folio)
>   			return NULL;
>   
> -		page_pincount_add(page, refs);
> -		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED,
> -				    refs);
> +		folio_pincount_add(folio, refs);
> +		node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, refs);
>   
> -		return page;
> +		return folio;
>   	}
>   
>   	WARN_ON_ONCE(1);
>   	return NULL;
>   }
>   
> +struct page *try_grab_compound_head(struct page *page,
> +		int refs, unsigned int flags)
> +{
> +	return &try_grab_folio(page, refs, flags)->page;
> +}
> +
>   static void put_compound_head(struct page *page, int refs, unsigned int flags)
>   {
>   	if (flags & FOLL_PIN) {
> @@ -196,7 +194,7 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
>    * @flags:   gup flags: these are the FOLL_* flag values.
>    *
>    * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
> - * time. Cases: please see the try_grab_compound_head() documentation, with
> + * time. Cases: please see the try_grab_folio() documentation, with
>    * "refs=1".
>    *
>    * Return: true for success, or if no action was required (if neither FOLL_PIN
> diff --git a/mm/internal.h b/mm/internal.h
> index 26af8a5a5be3..9a72d1ecdab4 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -723,4 +723,9 @@ void vunmap_range_noflush(unsigned long start, unsigned long end);
>   int numa_migrate_prep(struct page *page, struct vm_area_struct *vma,
>   		      unsigned long addr, int page_nid, int *flags);
>   
> +/*
> + * mm/gup.c
> + */
> +struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags);
> +
>   #endif	/* __MM_INTERNAL_H */