linux-kernel - [PATCH 3/3] binder: Use kmap_local_page() in binder_alloc_get

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Message-Id: <20220423102421.16869-4-fmdefrancesco@gmail.com>
Date:   Sat, 23 Apr 2022 12:24:21 +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
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Subject: [PATCH 3/3] binder: Use kmap_local_page() in binder_alloc_get_page()

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 it should be only used if it is absolutely
required, otherwise 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).

Therefore, use kmap_local_page() / kunmap_local() in place of
kmap_atomic() / kunmap_atomic().

Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---
 drivers/android/binder_alloc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 0875c463c002..058595cc7ff0 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -1250,17 +1250,17 @@ static int binder_alloc_do_buffer_copy(struct binder_alloc *alloc,
 		page = binder_alloc_get_page(alloc, buffer,
 					     buffer_offset, &pgoff);
 		size = min_t(size_t, bytes, PAGE_SIZE - pgoff);
-		base_ptr = kmap_atomic(page);
+		base_ptr = kmap_local_page(page);
 		tmpptr = base_ptr + pgoff;
 		if (to_buffer)
 			memcpy(tmpptr, ptr, size);
 		else
 			memcpy(ptr, tmpptr, size);
 		/*
-		 * kunmap_atomic() takes care of flushing the cache
+		 * kunmap_local() takes care of flushing the cache
 		 * if this device has VIVT cache arch
 		 */
-		kunmap_atomic(base_ptr);
+		kunmap_local(base_ptr);
 		bytes -= size;
 		pgoff = 0;
 		ptr = ptr + size;
-- 
2.34.1