Re: [PATCH v10 05/10] mm: zero reserved and unavailable struct pages

[Michal Hocko]

On Thu 05-10-17 17:11:19, 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).
> 
> 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().

As I've said in other reply this should go in only if the scenario you
describe is real. I am somehow suspicious to be honest. I simply do not
see how those weird struct pages would be in a valid pfn range of any
zone.

> Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Reviewed-by: Steven Sistare <steven.sistare@xxxxxxxxxx>
> Reviewed-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> Reviewed-by: Bob Picco <bob.picco@xxxxxxxxxx>
> ---
>  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