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Subject[net PATCH 1/2] mm: Use fixed constant in page_frag_alloc instead of size + 1
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From: Alexander Duyck <alexander.h.duyck@linux.intel.com>

This patch replaces the size + 1 value introduced with the recent fix for 1
byte allocs with a constant value.

The idea here is to reduce code overhead as the previous logic would have
to read size into a register, then increment it, and write it back to
whatever field was being used. By using a constant we can avoid those
memory reads and arithmetic operations in favor of just encoding the
maximum value into the operation itself.

Fixes: 2c2ade81741c ("mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs")
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
---
mm/page_alloc.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ebb35e4d0d90..37ed14ad0b59 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4857,11 +4857,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
/* Even if we own the page, we do not use atomic_set().
* This would break get_page_unless_zero() users.
*/
- page_ref_add(page, size);
+ page_ref_add(page, PAGE_FRAG_CACHE_MAX_SIZE);

/* reset page count bias and offset to start of new frag */
nc->pfmemalloc = page_is_pfmemalloc(page);
- nc->pagecnt_bias = size + 1;
+ nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
nc->offset = size;
}

@@ -4877,10 +4877,10 @@ void *page_frag_alloc(struct page_frag_cache *nc,
size = nc->size;
#endif
/* OK, page count is 0, we can safely set it */
- set_page_count(page, size + 1);
+ set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);

/* reset page count bias and offset to start of new frag */
- nc->pagecnt_bias = size + 1;
+ nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
offset = size - fragsz;
}

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