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

[Michal Hocko]

On Thu 12-10-17 10:19:16, Mike Kravetz wrote:
> On 10/12/2017 07:37 AM, Michal Hocko wrote:
> > On Wed 11-10-17 18:46:11, Mike Kravetz wrote:
> >> Add new MAP_CONTIG flag to mmap system call.  Check for flag in normal
> >> mmap flag processing.  If present, pre-allocate a contiguous set of
> >> pages to back the mapping.  These pages will be used a fault time, and
> >> the MAP_CONTIG flag implies populating the mapping at the mmap time.
> > 
> > I have only briefly read through the previous discussion and it is still
> > not clear to me _why_ we want such a interface. I didn't give it much
> > time yet but I do not think this is a good idea at all.
> 
> Thanks for looking Michal.  The primary use case comes from devices that can
> realize performance benefits if operating on physically contiguous memory.
> What sparked this effort was Christoph and Guy's plumbers presentation
> where they showed RDMA performance benefits that could be realized with
> contiguous memory.  I also remember sitting in a presentation about
> Intel's QuackAssist technology at Vault last year.  The presenter mentioned
> that their compression engine needed to be passed a physically contiguous
> buffer.  I asked how a user could obtain such a buffer.  They said they
> had a special driver/ioctl for that.  Yuck!  I'm guessing there are other
> specific use cases.  That is why I wanted to start the discussion as to
> whether there should be an interface to provide this functionality.

I would, quite contrary, suggest a device specific mmap implementation
which would guarantee both the best memory wrt. physical contiguous
aspect as well as the placement - what if the device have a restriction
on that as well?
 
> > any user to simply consume larger order memory blocks? What would
> > prevent from that?
> 
> We certainly would want to put restrictions in place for contiguous
> memory allocations.  Since it makes sense to pre-populate and lock
> contiguous allocations, using the same restrictions as mlock is a start.
> However, I can see the possible need for more restrictions.

Absolutely. mlock limit is per process (resp. mm) so a single user could
simply deplete large blocks. No good...
 
> > Does the memory always stays contiguous? How much contiguous it will be?
> 
> Yes, it remains contiguous.  It is locked in memory.

Hmm, so hugetlb on steroids...

> > Who is going to use such an interface? And probably many other
> > questions...
> 
> Thanks for asking.  I am just throwing out the idea of providing an interface
> for doing contiguous memory allocations from user space.  There are at least
> two (and possibly more) devices that could benefit from such an interface.

I am not really convinced this is a good interface. You are basically
trying to bypass virtual memory abstraction and that is quite
contradicting the mmap API to me.
-- 
Michal Hocko
SUSE Labs