linux-kernel - [PATCH v2 3/3] binder: Use memcpy_{to,from}_page() in binder_alloc_do_buffer

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Date:   Mon, 25 Apr 2022 19:57:54 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <brauner@...nel.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Ira Weiny <ira.weiny@...el.com>, linux-kernel@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Todd Kjos <tkjos@...gle.com>
Subject: [PATCH v2 3/3] binder: Use memcpy_{to,from}_page() in binder_alloc_do_buffer_copy()

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@...adoo.fr>
Acked-by: Todd Kjos <tkjos@...gle.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.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