linux-kernel - Re: [PATCH v2 1/2] mm/page_alloc: drop the unnecessary pfn

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Message-ID: <ZEZf1tbao9E8JFBr@dhcp22.suse.cz>
Date:   Mon, 24 Apr 2023 12:54:14 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, rppt@...nel.org, ying.huang@...el.com,
        mgorman@...hsingularity.net, vbabka@...e.cz, david@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/page_alloc: drop the unnecessary pfn_valid()
 for start pfn

On Mon 24-04-23 18:46:40, Baolin Wang wrote:
> 
> 
> On 4/24/2023 5:50 PM, Michal Hocko wrote:
> > On Sun 23-04-23 18:59:10, Baolin Wang wrote:
> > > We've already used pfn_to_online_page() for start pfn to make sure
> > 
> > Who is we? I do not see any note explicitly requiring that start_pfn has
> > to be valid for __pageblock_pfn_to_page.
> 
> Sorry for confusing, what I mean is the __pageblock_pfn_to_page() function,
> which has used pfn_to_online_page() for start pfn. So the pfn_valid() in
> __pageblock_pfn_to_page() for start pfn is unnecessary.
> 
> I will update the commit log to make it clear.

Your comment suggested that the check _has_ already been done. Which is
not the case. pfn_to_online_page is called later in the function so I
guess you should rephrase as following:

"
__pageblock_pfn_to_page currently performs both pfn_valid check and
pfn_to_online_page. The former one is redundant because the latter is a
stronger check. Drop pfn_valid.
"

With that or something going along with that. Feel free to add
Acked-by: Michal Hocko <mhocko@...e.com>

> > > it is online and valid, so the pfn_valid() for the start pfn is
> > > unnecessary, drop it.
> > > 
> > > Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> > > Reviewed-by: David Hildenbrand <david@...hat.com>
> > > Reviewed-by: "Huang, Ying" <ying.huang@...el.com>
> > > ---
> > > Changes from v1:
> > >   - Collect reviewed tags. Thanks David and Ying.
> > > ---
> > >   mm/page_alloc.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 9de2a18519a1..6457b64fe562 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -1512,7 +1512,7 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
> > >   	/* end_pfn is one past the range we are checking */
> > >   	end_pfn--;
> > > -	if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn))
> > > +	if (!pfn_valid(end_pfn))
> > >   		return NULL;
> > >   	start_page = pfn_to_online_page(start_pfn);
> > > -- 
> > > 2.27.0
> > 

-- 
Michal Hocko
SUSE Labs