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

[Baolin Wang]

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@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. 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 | 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.