Subject: Re: [PATCH] mm/sparse: Fix kernel crash with pfn_section_valid check

From: Baoquan He <[email-masked]>
Date: Wed, 25 Mar 2020 15:37:07 +0800

On 03/25/20 at 03:06pm, Baoquan He wrote:
> On 03/25/20 at 08:49am, Aneesh Kumar K.V wrote:

> >  mm/sparse.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/mm/sparse.c b/mm/sparse.c
> > index aadb7298dcef..3012d1f3771a 100644
> > --- a/mm/sparse.c
> > +++ b/mm/sparse.c
> > @@ -781,6 +781,8 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
> >  			ms->usage = NULL;
> >  		}
> >  		memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
> > +		/* Mark the section invalid */
> > +		ms->section_mem_map &= ~SECTION_HAS_MEM_MAP;
> 
> Not sure if we should add checking in valid_section() or pfn_valid(),
> e.g check ms->usage validation too. Otherwise, this fix looks good to
> me.

With SPASEMEM_VMEMAP enabled, we should do validation check on ms->usage
before checking any subsection is valid. Since now we do have case
in which ms->usage is released, people still try to check it.

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index f0a2c184eb9a..d79bd938852e 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1306,6 +1306,8 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
 {
 	int idx = subsection_map_index(pfn);
 
+	if (!ms->usage)
+		return 0;
 	return test_bit(idx, ms->usage->subsection_map);
 }
 #else