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

[Guy Shattah]

On 13/10/2017 19:17, Michal Hocko wrote:
> On Fri 13-10-17 10:56:13, Cristopher Lameter wrote:
> > On Fri, 13 Oct 2017, Michal Hocko wrote:
> >
> > > > There is a generic posix interface that could we used for a variety of
> > > > specific hardware dependent use cases.
> > > Yes you wrote that already and my counter argument was that this generic
> > > posix interface shouldn't bypass virtual memory abstraction.
> > It does do that? In what way?
> availability of the virtual address space depends on the availability of
> the same sized contiguous physical memory range. That sounds like the
> abstraction is gone to large part to me.
In what way? userspace users will still be working with virtual memory.

> > > > There are numerous RDMA devices that would all need the mmap
> > > > implementation. And this covers only the needs of one subsystem. There are
> > > > other use cases.
> > > That doesn't prevent providing a library function which could be reused
> > > by all those drivers. Nothing really too much different from
> > > remap_pfn_range.
> > And then in all the other use cases as well. It would be much easier if
> > mmap could give you the memory you need instead of havig numerous drivers
> > improvise on their own. This is in particular also useful
> > for numerous embedded use cases where you need contiguous memory.
> But a generic implementation would have to deal with many issues as
> already mentioned. If you make this driver specific you can have access
> control based on fd etc... I really fail to see how this is any
> different from remap_pfn_range.
Why have several driver specific implementation if you can generalize 
the idea and implement
an already existing POSIX standard?
--
Guy