On Wed, Oct 11, 2023 at 11:25:27AM -0400, Johannes Weiner wrote: > On Tue, Oct 10, 2023 at 05:12:01PM -0400, Johannes Weiner wrote: > > On Mon, Oct 02, 2023 at 10:26:44PM -0400, Zi Yan wrote: > > > @@ -1614,10 +1652,43 @@ static int move_freepages(struct zone *zone, unsigned long start_pfn, > > > > > > order = buddy_order(page); > > > move_to_free_list(page, zone, order, old_mt, new_mt); > > > + /* > > > + * set page migratetype 1) only after we move all free pages in > > > + * one pageblock and 2) for all pageblocks within the page. > > > + * > > > + * for 1), since move_to_free_list() checks page migratetype with > > > + * old_mt and changing one page migratetype affects all pages > > > + * within the same pageblock, if we are moving more than > > > + * one free pages in the same pageblock, setting migratetype > > > + * right after first move_to_free_list() triggers the warning > > > + * in the following move_to_free_list(). > > > + * > > > + * for 2), when a free page order is greater than pageblock_order, > > > + * all pageblocks within the free page need to be changed after > > > + * move_to_free_list(). > > > > I think this can be somewhat simplified. > > > > There are two assumptions we can make. Buddies always consist of 2^n > > pages. And buddies and pageblocks are naturally aligned. This means > > that if this pageblock has the start of a buddy that straddles into > > the next pageblock(s), it must be the first page in the block. That in > > turn means we can move the handling before the loop. > > Eh, scratch that. Obviously, a sub-block buddy can straddle blocks :( I apologize for the back and forth, but I think I had it right the first time. Say we have order-0 frees at pfn 511 and 512. Those can't merge because their order-0 buddies are 510 and 513 respectively. The same keeps higher-order merges below block size within the pageblock. So again, due to the pow2 alignment, the only way for a buddy to straddle a pageblock boundary is if it's >pageblock_order. Please double check me on this, because I've stared at your patches and the allocator code long enough now to thoroughly confuse myself. My proposal for the follow-up changes still stands for now.
