On Mon, 15 Mar 2021 10:42:05 +0000 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote: > On Sun, Mar 14, 2021 at 03:22:02PM +0000, Chuck Lever III wrote: > > >> Anyway, I'm not arguing against a bulk allocator, nor even saying this > > >> is a bad interface. It just maybe could be better. > > >> > > > > > > I think it puts more responsibility on the caller to use the API correctly > > > but I also see no value in arguing about it further because there is no > > > supporting data either way (I don't have routine access to a sufficiently > > > fast network to generate the data). I can add the following patch and let > > > callers figure out which interface is preferred. If one of the interfaces > > > is dead in a year, it can be removed. > > > > > > As there are a couple of ways the arrays could be used, I'm leaving it > > > up to Jesper and Chuck which interface they want to use. In particular, > > > it would be preferred if the array has no valid struct pages in it but > > > it's up to them to judge how practical that is. > > > > I'm interested to hear from Jesper. > > > > My two cents (US): > > > > If svc_alloc_arg() is the /only/ consumer that wants to fill > > a partially populated array of page pointers, then there's no > > code-duplication benefit to changing the synopsis of > > alloc_pages_bulk() at this point. > > > > Also, if the consumers still have to pass in the number of > > pages the array needs, rather than having the bulk allocator > > figure it out, then there's not much additional benefit, IMO. > > > > Ideally (for SUNRPC) alloc_pages_bulk() would take a pointer > > to a sparsely-populated array and the total number of elements > > in that array, and fill in the NULL elements. The return value > > would be "success -- all elements are populated" or "failure -- > > some elements remain NULL". > > > > If the array API interface was expected to handle sparse arrays, it would > make sense to define nr_pages are the number of pages that need to be > in the array instead of the number of pages to allocate. The preamble > would skip the first N number of allocated pages and decrement nr_pages > accordingly before the watermark check. The return value would then be the > last populated array element and the caller decides if that is enough to > proceed or if the API needs to be called again. There is a slight risk > that with a spare array that only needed 1 page in reality would fail > the watermark check but on low memory, allocations take more work anyway. > That definition of nr_pages would avoid the potential buffer overrun but > both you and Jesper would need to agree that it's an appropriate API. I actually like the idea of doing it this way. Even-though the page_pool fast-path (__page_pool_get_cached()) doesn't clear/mark the "consumed" elements with NULL. I'm ready to change page_pool to handle this when calling this API, as I think it will be faster than walking the linked list. Even-though my page_pool use-case doesn't have a sparse array to populate (like NFS/SUNRPC) then I can still use this API that Chuck is suggesting. Thus, I'm fine with this :-) (p.s. working on implementing Alexander Duyck's suggestions, but I don't have it ready yet, I will try to send new patch tomorrow. And I do realize that with this API change I have to reimplement it again, but as long as we make forward progress then I'll happily do it). -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer /* fast path */ static struct page *__page_pool_get_cached(struct page_pool *pool) { struct page *page; /* Caller MUST guarantee safe non-concurrent access, e.g. softirq */ if (likely(pool->alloc.count)) { /* Fast-path */ page = pool->alloc.cache[--pool->alloc.count]; } else { page = page_pool_refill_alloc_cache(pool); } return page; }
