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 kmapʼs 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>
Reviewed-by: Takashi Iwai <tiwai@xxxxxxx>
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@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);
> 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
>
