Re: [RFC PATCH 0/3] Use pageblock_order for cma and alloc_contig_range alignment.

On 11/23/21 17:35, Zi Yan wrote:

On 19 Nov 2021, at 10:15, Zi Yan wrote:

quoted

quoted

quoted

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.
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization