Re: [PATCH v11 5/9] mm: zero reserved and unavailable struct pages

[Michal Hocko <mhocko@kernel.org>]

On Tue 10-10-17 15:44:41, Michal Hocko wrote:
> On Mon 09-10-17 18:19:27, Pavel Tatashin wrote:
> > Some memory is reserved but unavailable: not present in memblock.memory
> > (because not backed by physical pages), but present in memblock.reserved.
> > Such memory has backing struct pages, but they are not initialized by going
> > through __init_single_page().
> > 
> > In some cases these struct pages are accessed even if they do not contain
> > any data. One example is page_to_pfn() might access page->flags if this is
> > where section information is stored (CONFIG_SPARSEMEM,
> > SECTION_IN_PAGE_FLAGS).
> > 
> > One example of such memory: trim_low_memory_range() unconditionally
> > reserves from pfn 0, but e820__memblock_setup() might provide the exiting
> > memory from pfn 1 (i.e. KVM).

Btw. I would add your example from http://lkml.kernel.org/r/bcf24369-ac37-cedd-a264-3396fb5cf39e@oracle.com
to do changelog
 
> > Since, struct pages are zeroed in __init_single_page(), and not during
> > allocation time, we must zero such struct pages explicitly.
> > 
> > The patch involves adding a new memblock iterator:
> > 	for_each_resv_unavail_range(i, p_start, p_end)
> > 
> > Which iterates through reserved && !memory lists, and we zero struct pages
> > explicitly by calling mm_zero_struct_page().
> > 
> > Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
> > Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
> > Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
> > Reviewed-by: Bob Picco <bob.picco@oracle.com>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> 
> > ---
> >  include/linux/memblock.h | 16 ++++++++++++++++
> >  include/linux/mm.h       | 15 +++++++++++++++
> >  mm/page_alloc.c          | 38 ++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 69 insertions(+)
> > 
> > diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> > index bae11c7e7bf3..ce8bfa5f3e9b 100644
> > --- a/include/linux/memblock.h
> > +++ b/include/linux/memblock.h
> > @@ -237,6 +237,22 @@ unsigned long memblock_next_valid_pfn(unsigned long pfn, unsigned long max_pfn);
> >  	for_each_mem_range_rev(i, &memblock.memory, &memblock.reserved,	\
> >  			       nid, flags, p_start, p_end, p_nid)
> >  
> > +/**
> > + * for_each_resv_unavail_range - iterate through reserved and unavailable memory
> > + * @i: u64 used as loop variable
> > + * @flags: pick from blocks based on memory attributes
> > + * @p_start: ptr to phys_addr_t for start address of the range, can be %NULL
> > + * @p_end: ptr to phys_addr_t for end address of the range, can be %NULL
> > + *
> > + * Walks over unavailable but reserved (reserved && !memory) areas of memblock.
> > + * Available as soon as memblock is initialized.
> > + * Note: because this memory does not belong to any physical node, flags and
> > + * nid arguments do not make sense and thus not exported as arguments.
> > + */
> > +#define for_each_resv_unavail_range(i, p_start, p_end)			\
> > +	for_each_mem_range(i, &memblock.reserved, &memblock.memory,	\
> > +			   NUMA_NO_NODE, MEMBLOCK_NONE, p_start, p_end, NULL)
> > +
> >  static inline void memblock_set_region_flags(struct memblock_region *r,
> >  					     unsigned long flags)
> >  {
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 065d99deb847..04c8b2e5aff4 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -94,6 +94,15 @@ extern int mmap_rnd_compat_bits __read_mostly;
> >  #define mm_forbids_zeropage(X)	(0)
> >  #endif
> >  
> > +/*
> > + * On some architectures it is expensive to call memset() for small sizes.
> > + * Those architectures should provide their own implementation of "struct page"
> > + * zeroing by defining this macro in <asm/pgtable.h>.
> > + */
> > +#ifndef mm_zero_struct_page
> > +#define mm_zero_struct_page(pp)  ((void)memset((pp), 0, sizeof(struct page)))
> > +#endif
> > +
> >  /*
> >   * Default maximum number of active map areas, this limits the number of vmas
> >   * per mm struct. Users can overwrite this number by sysctl but there is a
> > @@ -2001,6 +2010,12 @@ extern int __meminit __early_pfn_to_nid(unsigned long pfn,
> >  					struct mminit_pfnnid_cache *state);
> >  #endif
> >  
> > +#ifdef CONFIG_HAVE_MEMBLOCK
> > +void zero_resv_unavail(void);
> > +#else
> > +static inline void zero_resv_unavail(void) {}
> > +#endif
> > +
> >  extern void set_dma_reserve(unsigned long new_dma_reserve);
> >  extern void memmap_init_zone(unsigned long, int, unsigned long,
> >  				unsigned long, enum memmap_context);
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 20b0bace2235..5f0013bbbe9d 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -6209,6 +6209,42 @@ void __paginginit free_area_init_node(int nid, unsigned long *zones_size,
> >  	free_area_init_core(pgdat);
> >  }
> >  
> > +#ifdef CONFIG_HAVE_MEMBLOCK
> > +/*
> > + * Only struct pages that are backed by physical memory are zeroed and
> > + * initialized by going through __init_single_page(). But, there are some
> > + * struct pages which are reserved in memblock allocator and their fields
> > + * may be accessed (for example page_to_pfn() on some configuration accesses
> > + * flags). We must explicitly zero those struct pages.
> > + */
> > +void __paginginit zero_resv_unavail(void)
> > +{
> > +	phys_addr_t start, end;
> > +	unsigned long pfn;
> > +	u64 i, pgcnt;
> > +
> > +	/* Loop through ranges that are reserved, but do not have reported
> > +	 * physical memory backing.
> > +	 */
> > +	pgcnt = 0;
> > +	for_each_resv_unavail_range(i, &start, &end) {
> > +		for (pfn = PFN_DOWN(start); pfn < PFN_UP(end); pfn++) {
> > +			mm_zero_struct_page(pfn_to_page(pfn));
> > +			pgcnt++;
> > +		}
> > +	}
> > +
> > +	/*
> > +	 * Struct pages that do not have backing memory. This could be because
> > +	 * firmware is using some of this memory, or for some other reasons.
> > +	 * Once memblock is changed so such behaviour is not allowed: i.e.
> > +	 * list of "reserved" memory must be a subset of list of "memory", then
> > +	 * this code can be removed.
> > +	 */
> > +	pr_info("Reserved but unavailable: %lld pages", pgcnt);
> > +}
> > +#endif /* CONFIG_HAVE_MEMBLOCK */
> > +
> >  #ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
> >  
> >  #if MAX_NUMNODES > 1
> > @@ -6632,6 +6668,7 @@ void __init free_area_init_nodes(unsigned long *max_zone_pfn)
> >  			node_set_state(nid, N_MEMORY);
> >  		check_for_memory(pgdat, nid);
> >  	}
> > +	zero_resv_unavail();
> >  }
> >  
> >  static int __init cmdline_parse_core(char *p, unsigned long *core)
> > @@ -6795,6 +6832,7 @@ void __init free_area_init(unsigned long *zones_size)
> >  {
> >  	free_area_init_node(0, zones_size,
> >  			__pa(PAGE_OFFSET) >> PAGE_SHIFT, NULL);
> > +	zero_resv_unavail();
> >  }
> >  
> >  static int page_alloc_cpu_dead(unsigned int cpu)
> > -- 
> > 2.14.2
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs