On Wed, Dec 07, 2016 at 10:44:31AM -0800, Dave Hansen wrote: > On 12/07/2016 10:38 AM, Andrea Arcangeli wrote: > >> > and leaves room for the bitmap size to be encoded as well, if we decide > >> > we need a bitmap in the future. > > How would a bitmap ever be useful with very large page-order? > > Please, guys. Read the patches. *Please*. I did read the code but you didn't answer my question. Why should a feature exist in the code that will never be useful. Why do you think we could ever decide we'll need the bitmap in the future for high order pages? > The current code doesn't even _use_ a bitmap. It's not using it right now, my question is exactly when it will ever use it? Leaving the bitmap only for order 0 allocations when you already wiped all high pages orders from the buddy, doesn't seem very good idea overall as the chance you got order 0 pages with close physical address doesn't seem very high. It would be high if the loop that eat into every possible higher order didn't run first, but such loop just run and already wiped everything. Also note, we need to call compaction very aggressive before falling back from order 9 down to order 8. Ideally we should never use the page_shift = PAGE_SHIFT case at all! Which leaves the bitmap as best as an optimization for something that is suboptimal case already. If the bitmap starts to payoff it means the admin did a mistake and shrunk the guest too much. -- 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>