Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized memmaps in memory_failure()

[Naoya Horiguchi]

On Thu, Oct 10, 2019 at 09:17:42AM +0200, David Hildenbrand wrote:
> On 10.10.19 02:26, Naoya Horiguchi wrote:
> > On Wed, Oct 09, 2019 at 04:24:35PM +0200, David Hildenbrand wrote:
> >> We should check for pfn_to_online_page() to not access uninitialized
> >> memmaps. Reshuffle the code so we don't have to duplicate the error
> >> message.
> >>
> >> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> >> ---
> >>  mm/memory-failure.c | 14 ++++++++------
> >>  1 file changed, 8 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> >> index 7ef849da8278..e866e6e5660b 100644
> >> --- a/mm/memory-failure.c
> >> +++ b/mm/memory-failure.c
> >> @@ -1253,17 +1253,19 @@ int memory_failure(unsigned long pfn, int flags)
> >>  	if (!sysctl_memory_failure_recovery)
> >>  		panic("Memory failure on page %lx", pfn);
> >>  
> >> -	if (!pfn_valid(pfn)) {
> >> +	p = pfn_to_online_page(pfn);
> >> +	if (!p) {
> >> +		if (pfn_valid(pfn)) {
> >> +			pgmap = get_dev_pagemap(pfn, NULL);
> >> +			if (pgmap)
> >> +				return memory_failure_dev_pagemap(pfn, flags,
> >> +								  pgmap);
> >> +		}
> >>  		pr_err("Memory failure: %#lx: memory outside kernel control\n",
> >>  			pfn);
> >>  		return -ENXIO;
> >>  	}
> >>  
> >> -	pgmap = get_dev_pagemap(pfn, NULL);
> >> -	if (pgmap)
> >> -		return memory_failure_dev_pagemap(pfn, flags, pgmap);
> >> -
> >> -	p = pfn_to_page(pfn);
> > 
> > This change seems to assume that memory_failure_dev_pagemap() is never
> > called for online pages. Is it an intended behavior?
> > Or the concept "online pages" is not applicable to zone device pages?
> 
> Yes, that's the real culprit. ZONE_DEVICE/devmem pages are never online
> (SECTION_IS_ONLINE). The terminology "online" only applies to pages that
> were given to the buddy. And as we support sup-section hotadd for
> devmem, we cannot easily make use of the section flag it. I already
> proposed somewhere to convert SECTION_IS_ONLINE to a subsection bitmap
> and call it something like pfn_active().
> 
> pfn_online() would then be "pfn_active() && zone != ZONE_DEVICE". And we
> could use pfn_active() everywhere to test for initialized memmaps (well,
> besides some special cases like device reserved memory that does not
> span full sub-sections). Until now, nobody volunteered and I have other
> things to do.

Thank you for explanation.  I'm not sure how hard now but we try to do this.
You fix the problem from online/offline viewpoint now, and leave remaining
part untouched. I agree with that approach, and the suggested change looks
good to me.

Acked-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>