Re: [PATCH v3 3/3] mm/compaction: optimize >0 order folio compaction with free page split.

[Vlastimil Babka <vbabka@suse.cz>]

On 2/9/24 20:57, Zi Yan wrote:
> On 9 Feb 2024, at 13:43, Vlastimil Babka wrote:
> 
>> On 2/2/24 17:15, 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 and premature compaction termination. 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 | 37 ++++++++++++++++++++++++++++++++++++-
>>>  1 file changed, 36 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>> index 58a4e3fb72ec..fa9993c8a389 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -1832,9 +1832,43 @@ 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_pages) {
>>> -		isolate_freepages(cc);
>>> +		int i;
>>> +
>>> +		for (i = order + 1; i < NR_PAGE_ORDERS; i++) {
>>
>> You could probably just start with a loop that finds the start_order (and do
>> the isolate_freepages() attempt if there's none) and then handle the rest
>> outside of the loop. No need to separately handle the case where you have
>> the exact order available?
> Like this?

Almost/

> if (list_empty(&cc->freepages[order].pages)) {

You don't need to do that under that if ().

>         int start_order;
> 
>         for (start_order = order + 1; start_order < NR_PAGE_ORDERS;

Just do start_order = order; ... (not order + 1).

The rest should just work.

>              start_order++)
>                 if (!list_empty(&cc->freepages[start_order].pages))
>                         break;
> 
>         /* no free pages in the list */
>         if (start_order == NR_PAGE_ORDERS) {
>                 if (!has_isolated_pages) {
>                         isolate_freepages(cc);
>                         has_isolated_pages = true;
>                         goto again;
>                 } else
>                         return NULL;
>         }
> 
>         struct page *freepage =
>                 list_first_entry(&cc->freepages[start_order].pages,
>                                  struct page, lru);
> 
>         unsigned long size = 1 << start_order;
> 
>         list_del(&freepage->lru);
> 
>         while (start_order > order) {
>                 start_order--;
>                 size >>= 1;
> 
>                 list_add(&freepage[size].lru,
>                         &cc->freepages[start_order].pages);
>                 set_page_private(&freepage[size], start_order);
>         }
>         dst = (struct folio *)freepage;
>         goto done;
> }
> 
>>
>>> +			if (cc->freepages[i].nr_pages) {
>>> +				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_pages--;
>>> +
>>> +				while (start_order > order) {
>>
>> With exact order available this while loop will just be skipped and that's
>> all the difference to it?
>>
>>> +					start_order--;
>>> +					size >>= 1;
>>> +
>>> +					list_add(&freepage[size].lru,
>>> +						&cc->freepages[start_order].pages);
>>> +					cc->freepages[start_order].nr_pages++;
>>> +					set_page_private(&freepage[size], start_order);
>>> +				}
>>> +				dst = (struct folio *)freepage;
>>> +				goto done;
>>> +			}
>>> +		}
>>> +		if (!has_isolated_pages) {
>>> +			isolate_freepages(cc);
>>> +			has_isolated_pages = true;
>>> +			goto again;
>>> +		}
>>> +
>>>  		if (!cc->freepages[order].nr_pages)
>>>  			return NULL;
>>>  	}
>>> @@ -1842,6 +1876,7 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
>>>  	dst = list_first_entry(&cc->freepages[order].pages, struct folio, lru);
>>>  	cc->freepages[order].nr_pages--;
>>>  	list_del(&dst->lru);
>>> +done:
>>>  	post_alloc_hook(&dst->page, order, __GFP_MOVABLE);
>>>  	if (order)
>>>  		prep_compound_page(&dst->page, order);
> 
> 
> --
> Best Regards,
> Yan, Zi