On 11/29/21 23:08, Zi Yan wrote: > 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? Yes. > How about MIGRATE_HIGHATOMIC? It should not be merged with other migratetypes > from my understanding. Hmm it shouldn't minimally because it has an accounting that would become broken. So it should prevent merging or make sure the reservations are with MAX_ORDER granularity, but seems that neither is true? CCing Mel. > -- > Best Regards, > Yan, Zi >
