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

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Date:   Fri, 15 Jul 2022 08:59:41 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     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,
        Ira Weiny <ira.weiny@...el.com>,
        kernel test robot <lkp@...el.com>
Subject: 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@...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>

Reviewed-by: Takashi Iwai <tiwai@...e.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@...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
>