linux-kernel - Re: [PATCH] mm, page_alloc: check pfn is valid before moving to freelist

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Date:   Tue, 12 Apr 2022 14:05:51 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH] mm, page_alloc: check pfn is valid before moving to
 freelist

On Tue, 12 Apr 2022, Andrew Morton wrote:

> On Tue, 12 Apr 2022 13:16:23 -0700 Sudarshan Rajagopalan <quic_sudaraja@...cinc.com> wrote:
> 
> > Check if pfn is valid before or not before moving it to freelist.
> > 
> > There are possible scenario where a pageblock can have partial physical
> > hole and partial part of System RAM. This happens when base address in RAM
> > partition table is not aligned to pageblock size.
> > 
> > ...
> >
> > 
> > Signed-off-by: Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>
> > Fixes: 4c7b9896621be ("mm: remove pfn_valid_within() and CONFIG_HOLES_IN_ZONE")
> 
> I made that 859a85ddf90e714092dea71a0e54c7b9896621be and added
> cc:stable.  I'll await reviewer input before proceeding further.
> 

Acked-by: David Rientjes <rientjes@...gle.com>

> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2521,6 +2521,11 @@ static int move_freepages(struct zone *zone,
> >  	int pages_moved = 0;
> >  
> >  	for (pfn = start_pfn; pfn <= end_pfn;) {
> > +		if (!pfn_valid(pfn)) {
> 
> Readers will wonder how we can encounter an invalid pfn here.  A small
> comment might help clue them in.
> 

Sudarshan can correct me if I'm wrong, but this has to do with the 
pageblock alignment of the caller that assumes all pages in the range has 
an underlying struct page that we can access but that fails to hold true 
when we have a memory hole.  A comment would definitely help:

	/* Pageblock alignment may cause us to try to access into a hole */

> > +			pfn++;
> > +			continue;
> > +		}
> > +
> >  		page = pfn_to_page(pfn);
> >  		if (!PageBuddy(page)) {
> >  			/*
> 
> 
>