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Subject[PATCH v2 3/3] binder: Use memcpy_{to,from}_page() in binder_alloc_do_buffer_copy()
Date
The use of kmap_atomic() is being deprecated in favor of kmap_local_page()
where it is feasible. Each call of kmap_atomic() in the kernel creates
a non-preemptible section and disable pagefaults. This could be a source
of unwanted latency, so kmap_local_page() should be preferred.

With kmap_local_page(), the mapping is per thread, CPU local and not
globally visible. Furthermore, the mapping can be acquired from any context
(including interrupts). binder_alloc_do_buffer_copy() is a function where
the use of kmap_local_page() in place of kmap_atomic() is correctly suited.

Use kmap_local_page() / kunmap_local() in place of kmap_atomic() /
kunmap_atomic() but, instead of open coding the mappings and call memcpy()
to and from the virtual addresses of the mapped pages, prefer the use of
the memcpy_{to,from}_page() wrappers (as suggested by Christophe
Jaillet).

Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---

v1->v2: Add Todd Kjos's tag in the commit message (thanks!); re-write
the subject which referred to another function (my fault!); use the
memcpy_{to,from}_page() (as suggested by Christophe - thanks!); extend
and rework the commit message to say something about the use of the
above-mentioned wrappers.

drivers/android/binder_alloc.c | 13 ++-----------
1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 0875c463c002..5649a0371a1f 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -1244,23 +1244,14 @@ static int binder_alloc_do_buffer_copy(struct binder_alloc *alloc,
unsigned long size;
struct page *page;
pgoff_t pgoff;
- void *tmpptr;
- void *base_ptr;

page = binder_alloc_get_page(alloc, buffer,
buffer_offset, &pgoff);
size = min_t(size_t, bytes, PAGE_SIZE - pgoff);
- base_ptr = kmap_atomic(page);
- tmpptr = base_ptr + pgoff;
if (to_buffer)
- memcpy(tmpptr, ptr, size);
+ memcpy_to_page(page, pgoff, ptr, size);
else
- memcpy(ptr, tmpptr, size);
- /*
- * kunmap_atomic() takes care of flushing the cache
- * if this device has VIVT cache arch
- */
- kunmap_atomic(base_ptr);
+ memcpy_from_page(ptr, page, pgoff, size);
bytes -= size;
pgoff = 0;
ptr = ptr + size;
--
2.34.1
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