Re: [rfc][patch 0/4] VM_MIXEDMAP patchset with s390 backend

[Carsten Otte <cotte@de.ibm.com>]

Nick Piggin wrote:
> I'm just curious (or forgetful) as to why s390's pfn_valid does not walk
> your memory segments? (That would allow the s390 proof of concept to be
> basically a noop, and mixedmap_refcount_pfn will only be required when
> we start using another pte bit.
Our pfn_valid uses a hardware instruction, which does check if there 
is memory behind a pfn which we can access. And we'd like to use the 
very same memory segment for both regular memory hotplug where the 
memory ends up in ZONE_NORMAL (in this case the memory would be 
read+write, and not shared with other guests), and for backing xip 
file systems (in this case the memory would be read-only, and shared). 
And in both cases, our instruction does consider the pfn to be valid. 
Thus, pfn_valid is not the right indicator for us to check if we need 
refcounting or not.

> I think using another bit in the pte for special mappings is reasonable.
> As I posted in my earlier patch, we can also use it to simplify vm_normal_page,
> and it facilitates a lock free get_user_pages.
That patch looks very nice. I am going to define PTE_SPECIAL for s390 
arch next...

> Anyway, hmm... I guess we should probably get these patches into -mm and
> then upstream soon. Any objections from anyone? Do you guys have performance /
> stress testing for xip?
I think it is mature enough to push upstream, I've booted a distro 
with /usr on it. But I really really want to exchange patch #4 with a 
pte-bit based one before pushing this.

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