linux-kernel - [PATCH v2] firmware_loader: Replace kmap() with kmap_local

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Message-Id: <20220714235030.12732-1-fmdefrancesco@gmail.com>
Date:   Fri, 15 Jul 2022 01:50:30 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Shuah Khan <shuah@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Takashi Iwai <tiwai@...e.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Tianfei zhang <tianfei.zhang@...el.com>,
        Russ Weight <russell.h.weight@...el.com>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>,
        kernel test robot <lkp@...el.com>
Subject: [PATCH v2] firmware_loader: Replace kmap() with kmap_local_page()

The use of kmap() is being deprecated in favor of kmap_local_page().

Two main problems with kmap(): (1) It comes with an overhead as mapping
space is restricted and protected by a global lock for synchronization and
(2) kmap() also requires global TLB invalidation when the kmap’s pool
wraps and it might block when the mapping space is fully utilized until a
slot becomes available.

kmap_local_page() is preferred over kmap() and kmap_atomic(). Where it
cannot mechanically replace the latters, code refactor should be considered
(special care must be taken if kernel virtual addresses are aliases in
different contexts).

With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts).

Call kmap_local_page() in firmware_loader wherever kmap() is currently
used. In firmware_rw() use the helpers copy_{from,to}_page() instead of
open coding the local mappings + memcpy().

Successfully tested with "firmware" selftests on a QEMU/KVM 32-bits VM
with 4GB RAM, booting a kernel with HIGHMEM64GB enabled.

Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Luis Chamberlain <mcgrof@...nel.org>
Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---

v1->v2: According to the comments from Greg Kroah-Hartman (thanks!),
extend the commit message adding information about why kmap() should be
avoided. Delete an unused variable left in the code of v1, which has been
Reported-by: kernel test robot <lkp@...el.com>

 drivers/base/firmware_loader/main.c  |  4 ++--
 drivers/base/firmware_loader/sysfs.c | 10 ++++------
 2 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
index ac3f34e80194..7c3590fd97c2 100644
--- a/drivers/base/firmware_loader/main.c
+++ b/drivers/base/firmware_loader/main.c
@@ -435,11 +435,11 @@ static int fw_decompress_xz_pages(struct device *dev, struct fw_priv *fw_priv,

 		/* decompress onto the new allocated page */
 		page = fw_priv->pages[fw_priv->nr_pages - 1];
-		xz_buf.out = kmap(page);
+		xz_buf.out = kmap_local_page(page);
 		xz_buf.out_pos = 0;
 		xz_buf.out_size = PAGE_SIZE;
 		xz_ret = xz_dec_run(xz_dec, &xz_buf);
-		kunmap(page);
+		kunmap_local(xz_buf.out);
 		fw_priv->size += xz_buf.out_pos;
 		/* partial decompression means either end or error */
 		if (xz_buf.out_pos != PAGE_SIZE)
diff --git a/drivers/base/firmware_loader/sysfs.c b/drivers/base/firmware_loader/sysfs.c
index 5b0b85b70b6f..77bad32c481a 100644
--- a/drivers/base/firmware_loader/sysfs.c
+++ b/drivers/base/firmware_loader/sysfs.c
@@ -242,19 +242,17 @@ static void firmware_rw(struct fw_priv *fw_priv, char *buffer,
 			loff_t offset, size_t count, bool read)
 {
 	while (count) {
-		void *page_data;
 		int page_nr = offset >> PAGE_SHIFT;
 		int page_ofs = offset & (PAGE_SIZE - 1);
 		int page_cnt = min_t(size_t, PAGE_SIZE - page_ofs, count);

-		page_data = kmap(fw_priv->pages[page_nr]);
-
 		if (read)
-			memcpy(buffer, page_data + page_ofs, page_cnt);
+			memcpy_from_page(buffer, fw_priv->pages[page_nr],
+					 page_ofs, page_cnt);
 		else
-			memcpy(page_data + page_ofs, buffer, page_cnt);
+			memcpy_to_page(fw_priv->pages[page_nr], page_ofs,
+				       buffer, page_cnt);

-		kunmap(fw_priv->pages[page_nr]);
 		buffer += page_cnt;
 		offset += page_cnt;
 		count -= page_cnt;
-- 
2.37.0