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

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Date:   Wed, 13 Apr 2022 23:55:59 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        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, Apr 12, 2022 at 02:05:51PM -0700, David Rientjes wrote:
> 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:

We do have a struct page for every page in a pageblock even if there is a
hole in the physical memory. If this is not the case, there is more
fundamental bug that should be fixed.
 
> 	/* Pageblock alignment may cause us to try to access into a hole */
> 
> > > +			pfn++;
> > > +			continue;
> > > +		}
> > > +
> > >  		page = pfn_to_page(pfn);
> > >  		if (!PageBuddy(page)) {
> > >  			/*
> > 
> > 
> > 

-- 
Sincerely yours,
Mike.