linux-kernel - Re: [PATCH v3 0/7] binder: eliminate use of vmalloc space for binder buffers

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Date:   Mon, 11 Feb 2019 08:57:08 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Todd Kjos <tkjos@...roid.com>
Cc:     tkjos@...gle.com, gregkh@...uxfoundation.org, arve@...roid.com,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        maco@...gle.com, joel@...lfernandes.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 0/7] binder: eliminate use of vmalloc space for binder
 buffers

On Fri, Feb 08, 2019 at 10:35:13AM -0800, Todd Kjos wrote:
> Binder buffers have always been mapped into kernel space
> via map_kernel_range_noflush() to allow the binder driver
> to modify the buffer before posting to userspace for
> processing.
> 
> In recent Android releases, the number of long-running
> binder processes has increased to the point that for
> 32-bit systems, there is a risk of running out of
> vmalloc space.
> 
> This patch set removes the persistent mapping of the
> binder buffers into kernel space. Instead, the binder
> driver creates temporary mappings with kmap() or
> kmap_atomic() to copy to or from the buffer only when
> necessary.

Is there any good reason to actually map the user memory to kernel
space instead of just using copy_{to,from}_user?