On 23 Nov 2021, at 12:32, Vlastimil Babka wrote: > 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. Thanks for the explanation. Basically migratetypes that can fall back to each other can be merged into a single free page, right? How about MIGRATE_HIGHATOMIC? It should not be merged with other migratetypes from my understanding. -- Best Regards, Yan, Zi
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