[PATCH v2] firmware_loader: Replace kmap() with kmap_local_page()

From: Fabio M. De Francesco
Date: Thu Jul 14 2022 - 19:50:53 EST

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@xxxxxxxxxxxxxxxxxxx>
Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx>
Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>

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@xxxxxxxxx>

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);
- 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;