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

[Zi Yan]

On 9 Feb 2024, at 13:43, Vlastimil Babka wrote:

> On 2/2/24 17:15, Zi Yan wrote:
>> From: Zi Yan <ziy@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> ---
>>  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?

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

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

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