linux-kernel - Re: [PATCH v2] mm: huge_memory: convert split_huge_pages

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Message-Id: <20230105190658.f5d9a4bc9619ad6bec91a973@linux-foundation.org>
Date:   Thu, 5 Jan 2023 19:06:58 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <willy@...radead.org>
Subject: Re: [PATCH v2] mm: huge_memory: convert split_huge_pages_all() to
 use a folio

On Tue, 3 Jan 2023 09:58:36 +0800 Kefeng Wang <wangkefeng.wang@...wei.com> wrote:

> 
> 
> On 2022/12/31 5:45, Andrew Morton wrote:
> > On Fri, 30 Dec 2022 17:30:20 +0800 Kefeng Wang <wangkefeng.wang@...wei.com> 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.

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?