On 11/23/21 17:35, Zi Yan wrote: > On 19 Nov 2021, at 10:15, Zi Yan wrote: >>>> From what my understanding, cma required alignment of >>>> max(MAX_ORDER - 1, pageblock_order), because when MIGRATE_CMA was introduced, >>>> __free_one_page() does not prevent merging two different pageblocks, when >>>> MAX_ORDER - 1 > pageblock_order. But current __free_one_page() implementation >>>> does prevent that. >>> >>> But it does prevent that only for isolated pageblock, not CMA, and yout >>> patchset doesn't seem to expand that to CMA? Or am I missing something. >> >> Yeah, you are right. Originally, I thought preventing merging isolated pageblock >> with other types of pageblocks is sufficient, since MIGRATE_CMA is always >> converted from MIGRATE_ISOLATE. But that is not true. I will rework the code. >> Thanks for pointing this out. >> > > I find that two pageblocks with different migratetypes, like MIGRATE_RECLAIMABLE > and MIGRATE_MOVABLE can be merged into a single free page after I checked > __free_one_page() in detail and printed pageblock information during buddy page > merging. Yes, that can happen. I am not sure what consequence it will cause. Do you have any idea? For MIGRATE_RECLAIMABLE or MIGRATE_MOVABLE or even MIGRATE_UNMOVABLE it's absolutely fine. As long as these pageblocks are fully free (and they are if it's a single free page spanning 2 pageblocks), they can be of any of these type, as they can be reused as needed without causing fragmentation. But in case of MIGRATE_CMA and MIGRATE_ISOLATE, uncontrolled merging would break the specifics of those types. That's why the code is careful for MIGRATE_ISOLATE, and MIGRATE_CMA was until now done in MAX_ORDER granularity.
