Re: [PATCH] mm: free reserved area's memmap if possiable

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Mon, Feb 13, 2017 at 12:17:00PM +0000, Will Deacon wrote:
> Adding linux-arm-kernel and devicetree (look for "raw-pfn"), since this
> impacts directly on those.
> 
> On Mon, Feb 13, 2017 at 08:02:29PM +0800, zhouxianrong@huawei.com wrote:
> > From: zhouxianrong <zhouxianrong@huawei.com>
> > 
> > just like freeing no-map area's memmap we could free reserved
> > area's memmap as well only when user of reserved area indicate
> > that we can do this in dts or drivers. that is, user of reserved
> > area know how to use the reserved area who could not memblock_free
> > or free_reserved_xxx the reserved area and regard the area as raw
> > pfn usage. the patch supply a way to users who want to utilize the
> > memmap memory corresponding to raw pfn reserved areas as many as
> > possible.
> 
> I don't really understand this. Can you point me at a specific use-case,
> please? Is CMA involved here?

You don't need "dt permission" to free the memmap page array for the
regions between memory areas.  In fact, adding a DT property for that
goes against the "DT describes the hardware not the implementation"
requirement, since the memmap page array is a Linux implementation
detail.

32-bit ARM has been freeing the memmap page array between memory areas
for years since pre-DT days, and continues to do so.  See
free_unused_memmap().

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