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

On Fri, 15 Jul 2022 01:50:30 +0200, Fabio M. De Francesco wrote:

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@linuxfoundation.org Cc: Luis Chamberlain mcgrof@kernel.org Suggested-by: Ira Weiny ira.weiny@intel.com Signed-off-by: Fabio M. De Francesco fmdefrancesco@gmail.com

Reviewed-by: Takashi Iwai tiwai@suse.de

thanks,

Takashi

---

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@intel.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