Re: [RFC] mmap(MAP_CONTIG)

[Vlastimil Babka]

On 10/04/2017 01:56 AM, Mike Kravetz wrote:

Hi,

> At Plumbers this year, Guy Shattah and Christoph Lameter gave a presentation
> titled 'User space contiguous memory allocation for DMA' [1].  The slides

Hm I didn't find slides on that link, are they available?

> point out the performance benefits of devices that can take advantage of
> larger physically contiguous areas.
> 
> When such physically contiguous allocations are done today, they are done
> within drivers themselves in an ad-hoc manner.

As Michal N. noted, the drivers might have different requirements. Is
contiguity (without extra requirements) so common that it would benefit
from a userspace API change?
Also how are the driver-specific allocations done today? mmap() on the
driver's device? Maybe we could provide some in-kernel API/library to
make them less "ad-hoc". Conversion to MAP_ANONYMOUS would at first seem
like an improvement in that userspace would be able to use a generic
allocation API and all the generic treatment of anonymous pages (LRU
aging, reclaim, migration etc), but the restrictions you listed below
eliminate most of that?
(It's likely that I just don't have enough info about how it works today
so it's difficult to judge)

> In addition to allocations
> for DMA, allocations of this type are also performed for buffers used by
> coprocessors and other acceleration engines.
> 
> As mentioned in the presentation, posix specifies an interface to obtain
> physically contiguous memory.  This is via typed memory objects as described
> in the posix_typed_mem_open() man page.  Since Linux today does not follow
> the posix typed memory object model, adding infrastructure for contiguous
> memory allocations seems to be overkill.  Instead, a proposal was suggested
> to add support via a mmap flag: MAP_CONTIG.
> 
> mmap(MAP_CONTIG) would have the following semantics:
> - The entire mapping (length size) would be backed by physically contiguous
>   pages.
> - If 'length' physically contiguous pages can not be allocated, then mmap
>   will fail.
> - MAP_CONTIG only works with MAP_ANONYMOUS mappings.
> - MAP_CONTIG will lock the associated pages in memory.  As such, the same
>   privileges and limits that apply to mlock will also apply to MAP_CONTIG.
> - A MAP_CONTIG mapping can not be expanded.
> - At fork time, private MAP_CONTIG mappings will be converted to regular
>   (non-MAP_CONTIG) mapping in the child.  As such a COW fault in the child
>   will not require a contiguous allocation.
> 
> Some implementation considerations:
> - alloc_contig_range() or similar will be used for allocations larger
>   than MAX_ORDER.
> - MAP_CONTIG should imply MAP_POPULATE.  At mmap time, all pages for the
>   mapping must be 'pre-allocated', and they can only be used for the mapping,
>   so it makes sense to 'fault in' all pages.
> - Using 'pre-allocated' pages in the fault paths may be intrusive.
> - We need to keep keep track of those pre-allocated pages until the vma is
>   tore down, especially if free_contig_range() must be called.
> 
> Thoughts?
> - Is such an interface useful?
> - Any other ideas on how to achieve the same functionality?
> - Any thoughts on implementation?
> 
> I have started down the path of pre-allocating contiguous pages at mmap
> time and hanging those off the vma(vm_private_data) with some kludges to
> use the pages at fault time.  It is really ugly, which is why I am not
> sharing the code.  Hoping for some comments/suggestions.
> 
> [1] https://www.linuxplumbersconf.org/2017/ocw/proposals/4669
>