[PATCH v31 04/12] arm64: mm: allow for unmapping part of kernel mapping

[Mark Rutland]

On Thu, Feb 02, 2017 at 07:21:32PM +0900, AKASHI Takahiro wrote:
> On Wed, Feb 01, 2017 at 04:03:54PM +0000, Mark Rutland wrote:
> > Hi,
> > 
> > On Wed, Feb 01, 2017 at 09:46:23PM +0900, AKASHI Takahiro wrote:
> > > A new function, remove_pgd_mapping(), is added.
> > > It allows us to unmap a specific portion of kernel mapping later as far as
> > > the mapping is made using create_pgd_mapping() and unless we try to free
> > > a sub-set of memory range within a section mapping.
> > 
> > I'm not keen on adding more page table modification code. It was painful
> > enough to ensure that those worked in all configurations.
> > 
> > Why can't we reuse create_pgd_mapping()? If we pass page_mappings_only,
> > and use an invalid prot (i.e. 0), what is the problem?
> 
> As I did in v30?
> (though my implementation in v30 should be improved.)

Something like that. I wasn't entirely sure why we needed to change
those functions so much, so I'm clearly missing something there. I'll go
have another look.

> > I can see that we wouldn't free/reallocate the pud or pmd entries, but
> > the "wasted" memory should be small. If anything, I'd argue that it's
> > preferable to keep that around so that we don't have to allocate memory
> > when we need to map the crashkernel region.
> > 
> > Is there another problem I'm missing?
> 
> If we don't need to free unused page tables, that would make things
> much simple. There are still some minor problems on the merge, but
> we can sort it out.

I'm not sure I follow what you mean by 'on merge' here. Could you
elaborate?

Thanks,
Mark.