Re: [PATCH v2 4/5] mm/memory_hotplug: Make sure the pfn is aligned to the order when onlining

[Michal Hocko]

On Wed 14-08-19 18:09:16, David Hildenbrand wrote:
> On 14.08.19 17:41, David Hildenbrand wrote:
> > Commit a9cd410a3d29 ("mm/page_alloc.c: memory hotplug: free pages as higher
> > order") assumed that any PFN we get via memory resources is aligned to
> > to MAX_ORDER - 1, I am not convinced that is always true. Let's play safe,
> > check the alignment and fallback to single pages.
> > 
> > Cc: Arun KS <arunks@xxxxxxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Oscar Salvador <osalvador@xxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >  mm/memory_hotplug.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index 63b1775f7cf8..f245fb50ba7f 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -646,6 +646,9 @@ static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
> >  	 */
> >  	for (pfn = start_pfn; pfn < end_pfn; pfn += 1ul << order) {
> >  		order = min(MAX_ORDER - 1, get_order(PFN_PHYS(end_pfn - pfn)));
> > +		/* __free_pages_core() wants pfns to be aligned to the order */
> > +		if (unlikely(!IS_ALIGNED(pfn, 1ul << order)))
> > +			order = 0;
> >  		(*online_page_callback)(pfn_to_page(pfn), order);
> >  	}
> >  
> > 
> 
> @Michal, if you insist, we can drop this patch. "break first and fix
> later" is not part of my DNA :)

I do not insist but have already expressed that I am not a fan of this
change. Also I think that "break first" is quite an over statement here.

-- 
Michal Hocko
SUSE Labs