Re: [PATCH v5 2/3] mm/compaction: add support for >0 order folio memory compaction.

[Vlastimil Babka <vbabka@suse.cz>]

On 2/15/24 18:32, Zi Yan wrote:
> On 15 Feb 2024, at 11:57, Vlastimil Babka wrote:
> 
>> On 2/14/24 23:04, Zi Yan wrote:
>>> @@ -1849,10 +1857,22 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
>>>  static void compaction_free(struct folio *dst, unsigned long data)
>>>  {
>>>  	struct compact_control *cc = (struct compact_control *)data;
>>> +	int order = folio_order(dst);
>>> +	struct page *page = &dst->page;
>>> +
>>> +	if (folio_put_testzero(dst)) {
>>> +		free_pages_prepare_fpi_none(page, order);
>>> +
>>> +		INIT_LIST_HEAD(&dst->lru);
>>
>> (is this even needed? I think the state of first parameter of list_add() is
>> never expected to be in particular state?)
> 
> There is a __list_add_valid() performing list corruption checks.

Yes, but dst->lru becomes "new" in list_add() and __list_add_valid() and
those never check the contents of new, i.e. new->next or new->prev. We could
have done list_del(&dst->lru) which puts poison values there and then a
list_add() is fine. So dst->lru does not need the init, it's just confusing.
Init is for the list's list_head, not for the list entry.

>>>
>>> -	list_add(&dst->lru, &cc->freepages);
>>> -	cc->nr_freepages++;
>>> -	cc->nr_migratepages += 1 << folio_order(dst);
>>> +		list_add(&dst->lru, &cc->freepages[order]);
>>> +		cc->nr_freepages += 1 << order;
>>> +		cc->nr_migratepages += 1 << order;
>>
>> Hm actually this increment of nr_migratepages should happen even if we lost
>> the free page.
> 
> Because compaction_free() indicates the page is not migrated and nr_migratepages
> should be increased regardless.

Yes.

> Will fix it. Thanks.
> 
>>> +	}
>>> +	/*
>>> +	 * someone else has referenced the page, we cannot take it back to our
>>> +	 * free list.
>>> +	 */
>>>  }
> 
> 
> --
> Best Regards,
> Yan, Zi