At Plumbers this year, Guy Shattah and Christoph Lameter gave a presentation titled 'User space contiguous memory allocation for DMA' [1]. The slides 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. 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 -- Mike Kravetz -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
