[RFC PATCH v0] powerpc: Fix BUG_ON during memory unplug on radix

[Bharata B Rao]

On Wed, Jun 19, 2019 at 08:17:01PM +1000, Nicholas Piggin wrote:
> Bharata B Rao's on June 19, 2019 5:45 pm:
> > We hit the following BUG_ON when memory hotplugged before reboot
> > is unplugged after reboot:
> > 
> > kernel BUG at arch/powerpc/mm/pgtable-frag.c:113!
> > 
> >  remove_pagetable+0x594/0x6a0
> >  (unreliable)
> >  remove_pagetable+0x94/0x6a0
> >  vmemmap_free+0x394/0x410
> >  sparse_remove_one_section+0x26c/0x2e8
> >  __remove_pages+0x428/0x540
> >  arch_remove_memory+0xd0/0x170
> >  __remove_memory+0xd4/0x1a0
> >  dlpar_remove_lmb+0xbc/0x110
> >  dlpar_memory+0xa80/0xd20
> >  handle_dlpar_errorlog+0xa8/0x160
> >  pseries_hp_work_fn+0x2c/0x60
> >  process_one_work+0x46c/0x860
> >  worker_thread+0x364/0x5e0
> >  kthread+0x1b0/0x1c0
> >  ret_from_kernel_thread+0x5c/0x68
> > 
> > This occurs because, during reboot-after-hotplug, the hotplugged
> > memory range gets initialized as regular memory and page
> > tables are setup using memblock allocator. This means that we
> > wouldn't have initialized the PMD or PTE fragment count for
> > those PMD or PTE pages.
> > 
> > Fixing this includes 3 aspects:
> > 
> > - Walk the init_mm page tables from mem_init() and initialize
> >   the PMD and PTE fragment counts appropriately.
> > - When we do early allocation of PMD (and PGD as well) pages,
> >   allocate in page size PAGE_SIZE granularity so that we are
> >   sure that the complete page is available for us to set the
> >   fragment count which is part of struct page.
> > - When PMD or PTE page is freed, check if it comes from memblock
> >   allocator and free it appropriately.
> > 
> > Reported-by: Srikanth Aithal <sraithal at linux.vnet.ibm.com>
> > Signed-off-by: Bharata B Rao <bharata at linux.ibm.com>
> > ---
> >  arch/powerpc/include/asm/book3s/64/radix.h |  1 +
> >  arch/powerpc/include/asm/sparsemem.h       |  1 +
> >  arch/powerpc/mm/book3s64/pgtable.c         | 12 +++-
> >  arch/powerpc/mm/book3s64/radix_pgtable.c   | 67 +++++++++++++++++++++-
> >  arch/powerpc/mm/mem.c                      |  5 ++
> >  arch/powerpc/mm/pgtable-frag.c             |  5 +-
> >  6 files changed, 87 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/book3s/64/radix.h b/arch/powerpc/include/asm/book3s/64/radix.h
> > index 574eca33f893..4320f2790e8d 100644
> > --- a/arch/powerpc/include/asm/book3s/64/radix.h
> > +++ b/arch/powerpc/include/asm/book3s/64/radix.h
> > @@ -285,6 +285,7 @@ static inline unsigned long radix__get_tree_size(void)
> >  #ifdef CONFIG_MEMORY_HOTPLUG
> >  int radix__create_section_mapping(unsigned long start, unsigned long end, int nid);
> >  int radix__remove_section_mapping(unsigned long start, unsigned long end);
> > +void radix__fixup_pgtable_fragments(void);
> >  #endif /* CONFIG_MEMORY_HOTPLUG */
> >  #endif /* __ASSEMBLY__ */
> >  #endif
> > diff --git a/arch/powerpc/include/asm/sparsemem.h b/arch/powerpc/include/asm/sparsemem.h
> > index 3192d454a733..e662f9232d35 100644
> > --- a/arch/powerpc/include/asm/sparsemem.h
> > +++ b/arch/powerpc/include/asm/sparsemem.h
> > @@ -15,6 +15,7 @@
> >  #ifdef CONFIG_MEMORY_HOTPLUG
> >  extern int create_section_mapping(unsigned long start, unsigned long end, int nid);
> >  extern int remove_section_mapping(unsigned long start, unsigned long end);
> > +void fixup_pgtable_fragments(void);
> >  
> >  #ifdef CONFIG_PPC_BOOK3S_64
> >  extern int resize_hpt_for_hotplug(unsigned long new_mem_size);
> > diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
> > index 01bc9663360d..7efe9cc16b39 100644
> > --- a/arch/powerpc/mm/book3s64/pgtable.c
> > +++ b/arch/powerpc/mm/book3s64/pgtable.c
> > @@ -186,6 +186,13 @@ int __meminit remove_section_mapping(unsigned long start, unsigned long end)
> >  
> >  	return hash__remove_section_mapping(start, end);
> >  }
> > +
> > +void fixup_pgtable_fragments(void)
> > +{
> > +	if (radix_enabled())
> > +		radix__fixup_pgtable_fragments();
> > +}
> > +
> >  #endif /* CONFIG_MEMORY_HOTPLUG */
> >  
> >  void __init mmu_partition_table_init(void)
> > @@ -320,7 +327,10 @@ void pmd_fragment_free(unsigned long *pmd)
> >  	BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0);
> >  	if (atomic_dec_and_test(&page->pt_frag_refcount)) {
> >  		pgtable_pmd_page_dtor(page);
> > -		__free_page(page);
> > +		if (PageReserved(page))
> > +			free_reserved_page(page);
> 
> Hmm. Rather than adding this special case here, I wonder if you can
> just go along in your fixup walk and convert all these pages to
> non-reserved pages?
> 
> ClearPageReserved ; init_page_count ; adjust_managed_page_count ; 
> should do the trick, right?

Yes, that should. We are anyway fixing the frag count during
the walk, might as well do all the above too and avoid the special
case in the free path.

Regards,
Bharata.