On Fri, 3 Nov 2023 19:59:49 +0100 Petr Tesařík <petr@xxxxxxxxxxx> wrote: > > Not sure how to properly fix this as the different alignment > > requirements get pretty complex quickly. So would appreciate your > > input. > > I don't think it's possible to improve the allocation logic without > modifying the page allocator and/or the DMA atomic pool allocator to > take additional constraints into account. I don't understand. What speaks against calculating the amount of space needed, so that with the waste we can still fit the bounce-buffer in the pool? I believe alloc_size + combined_mask is a trivial upper bound, but we can do slightly better since we know that we allocate pages. For the sake of simplicity let us assume we only have the min_align_mask requirement. Then I believe the worst case is that we need (orig_addr & min_align_mask & PAGE_MASK) + (min_align_mask & ~PAGE_MASK) extra space to fit. Depending on how the semantics pan out one may be able to replace min_align_mask with combined_mask. Is your point that for large combined_mask values _get_free_pages(GFP_NOWAIT | __GFP_NOWARN, required_order) is not likely to complete successfully? Regards, Halil
