Re: [RFC PATCH 2/4] mm/compaction: optimize >0 order folio compaction with free page split.

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/19/2023 1:20 AM, Zi Yan wrote:
> On 18 Sep 2023, at 3:34, Baolin Wang wrote:
> 
>> On 9/13/2023 12:28 AM, Zi Yan wrote:
>>> From: Zi Yan <ziy@nvidia.com>
>>>
>>> During migration in a memory compaction, free pages are placed in an array
>>> of page lists based on their order. But the desired free page order (i.e.,
>>> the order of a source page) might not be always present, thus leading to
>>> migration failures. Split a high order free pages when source migration
>>> page has a lower order to increase migration successful rate.
>>>
>>> Note: merging free pages when a migration fails and a lower order free
>>> page is returned via compaction_free() is possible, but there is too much
>>> work. Since the free pages are not buddy pages, it is hard to identify
>>> these free pages using existing PFN-based page merging algorithm.
>>>
>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>> ---
>>>    mm/compaction.c | 40 +++++++++++++++++++++++++++++++++++++++-
>>>    1 file changed, 39 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>> index 868e92e55d27..45747ab5f380 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -1801,9 +1801,46 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
>>>    	struct compact_control *cc = (struct compact_control *)data;
>>>    	struct folio *dst;
>>>    	int order = folio_order(src);
>>> +	bool has_isolated_pages = false;
>>>   +again:
>>>    	if (!cc->freepages[order].nr_free) {
>>> -		isolate_freepages(cc);
>>> +		int i;
>>> +
>>> +		for (i = order + 1; i <= MAX_ORDER; i++) {
>>> +			if (cc->freepages[i].nr_free) {
>>> +				struct page *freepage =
>>> +					list_first_entry(&cc->freepages[i].pages,
>>> +							 struct page, lru);
>>> +
>>> +				int start_order = i;
>>> +				unsigned long size = 1 << start_order;
>>> +
>>> +				list_del(&freepage->lru);
>>> +				cc->freepages[i].nr_free--;
>>> +
>>> +				while (start_order > order) {
>>> +					start_order--;
>>> +					size >>= 1;
>>> +
>>> +					list_add(&freepage[size].lru,
>>> +						&cc->freepages[start_order].pages);
>>> +					cc->freepages[start_order].nr_free++;
>>> +					set_page_private(&freepage[size], start_order);
>>
>> IIUC, these split pages should also call functions to initialize? e.g. prep_compound_page()?
> 
> Not at this place. It is done right below and above "done" label. When free pages
> are on cc->freepages, we want to keep them without being post_alloc_hook() or
> prep_compound_page() processed for a possible future split. A free page is
> only initialized when it is returned by compaction_alloc().

Ah, I see. Thanks for explanation.