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

Christoph Hellwig
2017-12-29T07:50:16Z

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 <hch@lst.de>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
---
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 3df6cd4ffb40..891c77487a6a 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -507,22 +507,23 @@ 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 will 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 e0d82b6706d7..3affe7544b0c 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;