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

[Zi Yan <ziy@nvidia.com>]

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On 15 Feb 2024, at 15:02, Vlastimil Babka wrote:

> 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.

Got it. Will remove it.

>>>>
>>>> -	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


--
Best Regards,
Yan, Zi

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