diff mbox series

[v2,10/28] gup: Turn hpage_pincount_sub() into page_pincount_sub()

Message ID 20220110042406.499429-11-willy@infradead.org (mailing list archive)
State New
Headers show
Series Convert GUP to folios | expand

Commit Message

Matthew Wilcox (Oracle) Jan. 10, 2022, 4:23 a.m. UTC 
Remove an unnecessary VM_BUG_ON by handling pages both with and without
a pincount field in page_pincount_sub().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/gup.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

Comments

Christoph Hellwig Jan. 10, 2022, 8:32 a.m. UTC | #1 
On Mon, Jan 10, 2022 at 04:23:48AM +0000, Matthew Wilcox (Oracle) wrote:
> Remove an unnecessary VM_BUG_ON by handling pages both with and without
> a pincount field in page_pincount_sub().

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>
John Hubbard Jan. 11, 2022, 6:40 a.m. UTC | #2 
On 1/9/22 20:23, Matthew Wilcox (Oracle) wrote:
> Remove an unnecessary VM_BUG_ON by handling pages both with and without
> a pincount field in page_pincount_sub().
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>   mm/gup.c | 15 +++++++--------
>   1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 3ed9907f3c8d..aed48de3912e 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -48,12 +48,15 @@ static void page_pincount_add(struct page *page, int refs)
>   		page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
>   }
>   
> -static void hpage_pincount_sub(struct page *page, int refs)

OK, this is a correct transformation. But it does make the function, as
small as it is, rather hard to understand. It is now factored out at a
strange place, as evidenced in the difficulty in documenting it. Here's
my best attempt so far at documenting it here:

/*
  * Subtract @refs from the compound_pincount, if applicable. Otherwise, do
  * nothing. And in all cases, return the number of pages that should be passed
  * to put_page_refs().
  */

...which I think is worth adding, if we can't find a better factoring.

Either way, it's correct, so:


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


thanks,
diff mbox series

Patch

diff --git a/mm/gup.c b/mm/gup.c
index 3ed9907f3c8d..aed48de3912e 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -48,12 +48,15 @@  static void page_pincount_add(struct page *page, int refs)
 		page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
 }
 
-static void hpage_pincount_sub(struct page *page, int refs)
+static int page_pincount_sub(struct page *page, int refs)
 {
-	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
 	VM_BUG_ON_PAGE(page != compound_head(page), page);
 
-	atomic_sub(refs, compound_pincount_ptr(page));
+	if (hpage_pincount_available(page))
+		atomic_sub(refs, compound_pincount_ptr(page));
+	else
+		refs *= GUP_PIN_COUNTING_BIAS;
+	return refs;
 }
 
 /* Equivalent to calling put_page() @refs times. */
@@ -177,11 +180,7 @@  static void put_compound_head(struct page *page, int refs, unsigned int flags)
 	if (flags & FOLL_PIN) {
 		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED,
 				    refs);
-
-		if (hpage_pincount_available(page))
-			hpage_pincount_sub(page, refs);
-		else
-			refs *= GUP_PIN_COUNTING_BIAS;
+		refs = page_pincount_sub(page, refs);
 	}
 
 	put_page_refs(page, refs);