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Date:   Thu, 14 Apr 2022 17:37:27 -0700
From:   Ira Weiny <ira.weiny@intel.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc:     Mauro Carvalho Chehab <mchehab@kernel.org>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Hans Verkuil <hverkuil-cisco@xs4all.nl>,
        Tsuchiya Yuto <kitakar@gmail.com>,
        Martiros Shakhzadyan <vrzh@vrzh.net>,
        Hans de Goede <hdegoede@redhat.com>,
        linux-media@vger.kernel.org, linux-staging@lists.linux.dev,
        linux-kernel@vger.kernel.org, outreachy@lists.linux.dev
Subject: Re: [PATCH] staging: media: atomisp: Use kmap_local_page() in
 hmm_store()
Message-ID: <Yli+R7iLZKqO8kVP@iweiny-desk3>
References: <20220413225531.9425-1-fmdefrancesco@gmail.com>
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On Thu, Apr 14, 2022 at 12:55:31AM +0200, Fabio M. De Francesco wrote:
> The use of kmap() is being deprecated in favor of kmap_local_page()
> where it is feasible. The same is true for kmap_atomic().
> 
> In file pci/hmm/hmm.c, function hmm_store() test if we are in atomic
> context and, if so, it calls kmap_atomic(), if not, it calls kmap().
> 
> First of all, in_atomic() shouldn't be used in drivers. This macro
> cannot always detect atomic context; in particular, it cannot know
> about held spinlocks in non-preemptible kernels.
> 
> Notwithstanding what it is said above, this code doesn't need to care
> whether or not it is executing in atomic context. It can simply use
> kmap_local_page() / kunmap_local() that can instead do the mapping /
> unmapping regardless of the context.
> 
> With kmap_local_page(), the mapping is per thread, CPU local and not
> globally visible. Therefore, hmm_store()() is a function where the use
> of kmap_local_page() in place of both kmap() and kmap_atomic() is
> correctly suited.
> 
> Convert the calls of kmap() / kunmap() and kmap_atomic() /
> kunmap_atomic() to kmap_local_page() / kunmap_local() and drop the
> unnecessary tests which test if the code is in atomic context.
> 
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

I've had to research this a bit myself because I've not seen this pattern
before.

kmap_atomic() had 2 uses:

	1) a situation where the operation on the page requires pagefaults
	   and/or preemption to be disabled
	2) in a situation where one knows the code can't sleep

kmap_local_page() removes the second use case because kmap() is no longer the
only alternative; from the kdoc for kmap_atomic():

...
 * kmap_atomic - Atomically map a page for temporary usage - Deprecated!
...
 * Effectively a wrapper around kmap_local_page() which disables pagefaults
 * and preemption.
...

The deprecation is because any pagefault/preemption disabling should probably
be done explicitly from now on but allows for existing kmap_atomic() callers to
live on.

In this case, I suspect the original driver writer wanted to use kmap() but
hmm_store() was called from various contexts.  So they incorrectly tried to
protect against the (potentially) sleeping kmap() call.  In reality, I think
they could have simply called kmap_atomic() and skipped 'in_atomic()' check
altogether.

Regardless now that kmap_local_page() exists, this is correct.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

> ---
>  drivers/staging/media/atomisp/pci/hmm/hmm.c | 14 ++------------
>  1 file changed, 2 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/staging/media/atomisp/pci/hmm/hmm.c b/drivers/staging/media/atomisp/pci/hmm/hmm.c
> index 46ac082cd3f1..54188197c3dc 100644
> --- a/drivers/staging/media/atomisp/pci/hmm/hmm.c
> +++ b/drivers/staging/media/atomisp/pci/hmm/hmm.c
> @@ -482,10 +482,7 @@ int hmm_store(ia_css_ptr virt, const void *data, unsigned int bytes)
>  		idx = (virt - bo->start) >> PAGE_SHIFT;
>  		offset = (virt - bo->start) - (idx << PAGE_SHIFT);
>  
> -		if (in_atomic())
> -			des = (char *)kmap_atomic(bo->page_obj[idx].page);
> -		else
> -			des = (char *)kmap(bo->page_obj[idx].page);
> +		des = (char *)kmap_local_page(bo->page_obj[idx].page);
>  
>  		if (!des) {
>  			dev_err(atomisp_dev,
> @@ -512,14 +509,7 @@ int hmm_store(ia_css_ptr virt, const void *data, unsigned int bytes)
>  
>  		clflush_cache_range(des, len);
>  
> -		if (in_atomic())
> -			/*
> -			 * Note: kunmap_atomic requires return addr from
> -			 * kmap_atomic, not the page. See linux/highmem.h
> -			 */
> -			kunmap_atomic(des - offset);
> -		else
> -			kunmap(bo->page_obj[idx].page);
> +		kunmap_local(des);
>  	}
>  
>  	return 0;
> -- 
> 2.34.1
>