On 2/19/19 9:19 PM, Zi Yan wrote: > On 19 Feb 2019, at 19:18, Mike Kravetz wrote: >> Another high level question. One of the benefits of this approach is >> that exchanging pages does not require N free pages as you describe >> above. This assumes that the vma which we are trying to make contiguous >> is already populated. If it is not populated, then you also need to >> have N free pages. Correct? If this is true, then is the expected use >> case to first populate a vma, and then try to make contiguous? I would >> assume that if it is not populated and a request to make contiguous is >> given, we should try to allocate/populate the vma with contiguous pages >> at that time? > > Yes, I assume the pages within the VMA are already populated but not contiguous > yet. > > My approach considers memory contiguity as an on-demand resource. In some phases > of an application, accelerators or RDMA controllers would process/transfer data > in one > or more VMAs, at which time contiguous memory can help reduce address translation > overheads or lift certain constraints. And different VMAs could be processed at > different program phases, thus it might be hard to get contiguous memory for all > these VMAs at the allocation time using alloc_contig_pages(). My approach can > help get contiguous memory later, when the demand comes. > > For some cases, you definitely can use alloc_contig_pages() to give users > a contiguous area at page allocation time, if you know the user is going to use > this > area for accelerator data processing or as a RDMA buffer and the area size is > fixed. > > In addition, we can also use khugepaged approach, having a daemon periodically > scan VMAs and use alloc_contig_pages() to convert non-contiguous pages in a VMA > to contiguous pages, but it would require N free pages during the conversion. > > In sum, my approach complements alloc_contig_pages() and provides more flexibility. > It is not trying to replaces alloc_contig_pages(). Thank you for the explanation. That makes sense. I have mostly been thinking about contiguous memory from an allocation perspective and did not really consider other use cases. -- Mike Kravetz
