Re: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP_CONTIG) support

[David Nellans]

On 10/16/2017 12:42 PM, Michal Hocko wrote:
> On Mon 16-10-17 11:00:19, Cristopher Lameter wrote:
>> On Mon, 16 Oct 2017, Michal Hocko wrote:
>>> That being said, the list is far from being complete, I am pretty sure
>>> more would pop out if I thought more thoroughly. The bottom line is that
>>> while I see many problems to actually implement this feature and
>>> maintain it longterm I simply do not see a large benefit outside of a
>>> very specific HW.
>> There is not much new here in terms of problems. The hardware that
>> needs this seems to become more and more plentiful. That is why we need a
>> generic implementation.
> It would really help to name that HW and other potential usecases
> independent on the HW because I am rather skeptical about the
> _plentiful_ part. And so I really do not see any foundation to claim
> the generic part. Because, fundamentally, it is the HW which requires
> the specific memory placement/physically contiguous range etc. So the
> generic implementation doesn't really make sense in such a context.
>

There are TLB's in AMD Xen that can take advantage of contig memory to
improve TLB coverage. AFAIK contig is not functionally required, its
purely a performance optimization. Current Xen TLB implementation
doesn't support arbitrary contig lengths, page sizes, etc, but its a
start. This
type of TLB optimization can be handled on the back end by de-fragging
phys mem (when possible) now that both base and THPs can be easily
migrated; no need for up-front contig, but defrag isn't free either.