Re: [PATCH 3/6] mm: page_alloc: move free pages when converting block during isolation

[Zi Yan]

On 11 Sep 2023, at 16:47, Johannes Weiner wrote:

> On Mon, Sep 11, 2023 at 04:17:07PM -0400, Zi Yan wrote:
>> On 11 Sep 2023, at 15:41, Johannes Weiner wrote:
>>
>>> When claiming a block during compaction isolation, move any remaining
>>> free pages to the correct freelists as well, instead of stranding them
>>> on the wrong list. Otherwise, this encourages incompatible page mixing
>>> down the line, and thus long-term fragmentation.
>>>
>>> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>>> ---
>>>  mm/page_alloc.c | 5 ++++-
>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 3db405414174..f6f658c3d394 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -2548,9 +2548,12 @@ int __isolate_free_page(struct page *page, unsigned int order)
>>>  			 * Only change normal pageblocks (i.e., they can merge
>>>  			 * with others)
>>>  			 */
>>> -			if (migratetype_is_mergeable(mt))
>>> +			if (migratetype_is_mergeable(mt)) {
>>>  				set_pageblock_migratetype(page,
>>>  							  MIGRATE_MOVABLE);
>>> +				move_freepages_block(zone, page,
>>> +						     MIGRATE_MOVABLE, NULL);
>>> +			}
>>>  		}
>>>  	}
>>>
>>> -- 
>>> 2.42.0
>>
>> Is this needed?
>
> Yes, the problem is if we e.g. isolate half a block, then we'll
> convert the type of the whole block but strand the half we're not
> isolating. This can be a couple of hundred pages on the wrong list.
>
>> And is this correct?
>>
>> __isolate_free_page() removes the free page from a free list, but the added
>> move_freepages_block() puts the page back to another free list, making
>> __isolate_free_page() not do its work. OK. the for loop is going through
>> the pages within the pageblock, so move_freepages_block() should be used
>> on the rest of the pages on the pageblock.
>>
>> So to make this correct, the easies change might be move
>> del_page_from_free_list(page, zone, order) below this code chunk.
>
> There is a del_page_from_freelist() just above this diff hunk. That
> takes the page off the list and clears its PageBuddy.
>
> move_freepages_block() will then move only the remainder of the block
> that's still on the freelist with a mismatched type (move_freepages()
> only moves buddies).

Ah, I missed __ClearPageBuddy() in del_page_from_free_list(). Thanks.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>


--
Best Regards,
Yan, Zi

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