Re: [PATCH v2] mm: huge_memory: convert split_huge_pages_all() to use a folio

[Kefeng Wang]

On 2023/1/6 11:06, Andrew Morton wrote:
> On Tue, 3 Jan 2023 09:58:36 +0800 Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote:
>
> > On 2022/12/31 5:45, Andrew Morton wrote:
> > > On Fri, 30 Dec 2022 17:30:20 +0800 Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote:
> > >
> > > > Straightforwardly convert split_huge_pages_all() to use a folio.
> > > >
> > > > ...
> > > >
> > > > --- a/mm/huge_memory.c
> > > > +++ b/mm/huge_memory.c
> > > > @@ -2932,6 +2932,7 @@ static void split_huge_pages_all(void)
> > > >    {
> > > >    	struct zone *zone;
> > > >    	struct page *page;
> > > > +	struct folio *folio;
> > > >    	unsigned long pfn, max_zone_pfn;
> > > >    	unsigned long total = 0, split = 0;
> > > >    
> > > > @@ -2944,24 +2945,32 @@ static void split_huge_pages_all(void)
> > > >    			int nr_pages;
> > > >    
> > > >    			page = pfn_to_online_page(pfn);
> > > > -			if (!page || !get_page_unless_zero(page))
> > > > +			if (!page || PageTail(page))
> > > > +				continue;
> > > > +			folio = page_folio(page);
> > > > +			if (!folio_try_get(folio))
> > > >    				continue;
> > > >    
> > > > -			if (zone != page_zone(page))
> > > > +			if (unlikely(page_folio(page) != folio))
> > > > +				goto next;
> > > > +
> > > > +			if (zone != folio_zone(folio))
> > > >    				goto next;
> > >
> > > I'm still not understanding the above hunk.  Why is the
> > > "page_folio(page) != folio" check added?  Should it be commented?
> >
> > There is a comment in try_get_folio(), is it enough?
>
> Nobody would think to look at a private function in gup.c.

Sorry，I mean that comment is to explain why adding a new recheck.

> I suggest this folio_try_get() rule be documented at the
> folio_try_get() implementation site.
>
> Most callers do perform this check, but not the ones in vmscan.c ?

Hope Matthew could give some advise, thanks