On 02.12.20 16:49, Michal Hocko wrote: > On Wed 02-12-20 10:14:41, David Hildenbrand wrote: >> On 01.12.20 18:51, Minchan Kim wrote: >>> There is a need for special HW to require bulk allocation of >>> high-order pages. For example, 4800 * order-4 pages, which >>> would be minimum, sometimes, it requires more. >>> >>> To meet the requirement, a option reserves 300M CMA area and >>> requests the whole 300M contiguous memory. However, it doesn't >>> work if even one of those pages in the range is long-term pinned >>> directly or indirectly. The other option is to ask higher-order >> >> My latest knowledge is that pages in the CMA area are never long term >> pinned. >> >> https://lore.kernel.org/lkml/20201123090129.GD27488@xxxxxxxxxxxxxx/ >> >> "gup already tries to deal with long term pins on CMA regions and migrate >> to a non CMA region. Have a look at __gup_longterm_locked." >> >> We should rather identify ways how that is still possible and get rid of >> them. >> >> >> Now, short-term pinnings and PCP are other issues where >> alloc_contig_range() could be improved (e.g., in contrast to a FAST >> mode, a HARD mode which temporarily disables the PCP, ...). > > Agreed! > >>> size (e.g., 2M) than requested order(64K) repeatedly until driver >>> could gather necessary amount of memory. Basically, this approach >>> makes the allocation very slow due to cma_alloc's function >>> slowness and it could be stuck on one of the pageblocks if it >>> encounters unmigratable page. >>> >>> To solve the issue, this patch introduces cma_alloc_bulk. >>> >>> int cma_alloc_bulk(struct cma *cma, unsigned int align, >>> bool fast, unsigned int order, size_t nr_requests, >>> struct page **page_array, size_t *nr_allocated); >>> >>> Most parameters are same with cma_alloc but it additionally passes >>> vector array to store allocated memory. What's different with cma_alloc >>> is it will skip pageblocks without waiting/stopping if it has unmovable >>> page so that API continues to scan other pageblocks to find requested >>> order page. >>> >>> cma_alloc_bulk is best effort approach in that it skips some pageblocks >>> if they have unmovable pages unlike cma_alloc. It doesn't need to be >>> perfect from the beginning at the cost of performance. Thus, the API >>> takes "bool fast parameter" which is propagated into alloc_contig_range to >>> avoid significat overhead functions to inrecase CMA allocation success >>> ratio(e.g., migration retrial, PCP, LRU draining per pageblock) >>> at the cost of less allocation success ratio. If the caller couldn't >>> allocate enough, they could call it with "false" to increase success ratio >>> if they are okay to expense the overhead for the success ratio. >> >> Just so I understand what the idea is: >> >> alloc_contig_range() sometimes fails on CMA regions when trying to >> allocate big chunks (e.g., 300M). Instead of tackling that issue, you >> rather allocate plenty of small chunks, and make these small allocations >> fail faster/ make the allocations less reliable. Correct? >> >> I don't really have a strong opinion on that. Giving up fast rather than >> trying for longer sounds like a useful thing to have - but I wonder if >> it's strictly necessary for the use case you describe. >> >> I'd like to hear Michals opinion on that. > > Well, what I can see is that this new interface is an antipatern to our > allocation routines. We tend to control allocations by gfp mask yet you > are introducing a bool parameter to make something faster... What that > really means is rather arbitrary. Would it make more sense to teach > cma_alloc resp. alloc_contig_range to recognize GFP_NOWAIT, GFP_NORETRY resp. > GFP_RETRY_MAYFAIL instead? Minchan did that before, but I disliked gluing things like "don't drain lru, don't drain pcp" to GFP_NORETRY and shifting responsibility to the user. -- Thanks, David / dhildenb
