[12/17] mm: optimize dev_pagemap reference counting around get_dev_pagemap

Commit Message

Christoph Hellwig Dec. 15, 2017, 2:09 p.m. UTC

Change the calling convention so that get_dev_pagemap always consumes the
previous reference instead of doing this using an explicit earlier call to
put_dev_pagemap in the callers.

The callers will still need to put the final reference after finishing the
loop over the pages.

Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Logan Gunthorpe <[e-mail]>
---
 kernel/memremap.c | 17 +++++++++--------
 mm/gup.c          |  7 +++++--
 2 files changed, 14 insertions(+), 10 deletions(-)

Comments

Dan Williams Dec. 17, 2017, 5:28 p.m. UTC | #1

On Fri, Dec 15, 2017 at 6:09 AM, Christoph Hellwig <[e-mail]> wrote:
> Change the calling convention so that get_dev_pagemap always consumes the
> previous reference instead of doing this using an explicit earlier call to
> put_dev_pagemap in the callers.
>
> The callers will still need to put the final reference after finishing the
> loop over the pages.
>
> Signed-off-by: Christoph Hellwig <[e-mail]>
> Reviewed-by: Logan Gunthorpe <[e-mail]>
> ---
>  kernel/memremap.c | 17 +++++++++--------
>  mm/gup.c          |  7 +++++--
>  2 files changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index 43d94db97ff4..26764085785d 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -506,22 +506,23 @@ struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
>   * @pfn: page frame number to lookup page_map
>   * @pgmap: optional known pgmap that already has a reference
>   *
> - * @pgmap allows the overhead of a lookup to be bypassed when @pfn lands in the
> - * same mapping.
> + * If @pgmap is non-NULL and covers @pfn it will be returned as-is.  If @pgmap
> + * is non-NULL but does not cover @pfn the reference to it while be released.

s/while/will/


Other than that you can add:

Reviewed-by: Dan Williams <[e-mail]>

Patch

diff --git a/kernel/memremap.c b/kernel/memremap.c
index 43d94db97ff4..26764085785d 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -506,22 +506,23 @@  struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
  * @pfn: page frame number to lookup page_map
  * @pgmap: optional known pgmap that already has a reference
  *
- * @pgmap allows the overhead of a lookup to be bypassed when @pfn lands in the
- * same mapping.
+ * If @pgmap is non-NULL and covers @pfn it will be returned as-is.  If @pgmap
+ * is non-NULL but does not cover @pfn the reference to it while be released.
  */
 struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
 		struct dev_pagemap *pgmap)
 {
-	const struct resource *res = pgmap ? pgmap->res : NULL;
 	resource_size_t phys = PFN_PHYS(pfn);
 
 	/*
-	 * In the cached case we're already holding a live reference so
-	 * we can simply do a blind increment
+	 * In the cached case we're already holding a live reference.
 	 */
-	if (res && phys >= res->start && phys <= res->end) {
-		percpu_ref_get(pgmap->ref);
-		return pgmap;
+	if (pgmap) {
+		const struct resource *res = pgmap ? pgmap->res : NULL;
+
+		if (res && phys >= res->start && phys <= res->end)
+			return pgmap;
+		put_dev_pagemap(pgmap);
 	}
 
 	/* fall back to slow path lookup */
diff --git a/mm/gup.c b/mm/gup.c
index d3fb60e5bfac..9d142eb9e2e9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1410,7 +1410,6 @@  static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 
 		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 
-		put_dev_pagemap(pgmap);
 		SetPageReferenced(page);
 		pages[*nr] = page;
 		(*nr)++;
@@ -1420,6 +1419,8 @@  static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 	ret = 1;
 
 pte_unmap:
+	if (pgmap)
+		put_dev_pagemap(pgmap);
 	pte_unmap(ptem);
 	return ret;
 }
@@ -1459,10 +1460,12 @@  static int __gup_device_huge(unsigned long pfn, unsigned long addr,
 		SetPageReferenced(page);
 		pages[*nr] = page;
 		get_page(page);
-		put_dev_pagemap(pgmap);
 		(*nr)++;
 		pfn++;
 	} while (addr += PAGE_SIZE, addr != end);
+
+	if (pgmap)
+		put_dev_pagemap(pgmap);
 	return 1;
 }