Re: [PATCH] mm: don't do zero_resv_unavail if memmap is not allocated

Michal Hocko
2018-07-16T16:09:59Z

On Mon 16-07-18 11:16:30, Pavel Tatashin wrote:
> Moving zero_resv_unavail before memmap_init_zone(), caused a regression on
> x86-32.
>
> The cause is that we access struct pages before they are allocated when
> CONFIG_FLAT_NODE_MEM_MAP is used.
>
> free_area_init_nodes()
> zero_resv_unavail()
> mm_zero_struct_page(pfn_to_page(pfn)); <- struct page is not alloced
> free_area_init_node()
> if CONFIG_FLAT_NODE_MEM_MAP
> alloc_node_mem_map()
> memblock_virt_alloc_node_nopanic() <- struct page alloced here
>
> On the other hand memblock_virt_alloc_node_nopanic() zeroes all the memory
> that it returns, so we do not need to do zero_resv_unavail() here.

This all is subtle as hell and almost impossible to build a sane code on
top. Your patch sounds good as a stop gap fix but we really need
something resembling an actual design rather than ad-hoc hacks piled on
top of each other.

> Fixes: e181ae0c5db9 ("mm: zero unavailable pages before memmap init")
> Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> include/linux/mm.h | 2 +-
> mm/page_alloc.c | 4 ++--
> 2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a0fbb9ffe380..3982c83fdcbf 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2132,7 +2132,7 @@ extern int __meminit __early_pfn_to_nid(unsigned long pfn,
> struct mminit_pfnnid_cache *state);
> #endif
>
> -#ifdef CONFIG_HAVE_MEMBLOCK
> +#if defined(CONFIG_HAVE_MEMBLOCK) && !defined(CONFIG_FLAT_NODE_MEM_MAP)
> void zero_resv_unavail(void);
> #else
> static inline void zero_resv_unavail(void) {}
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5d800d61ddb7..a790ef4be74e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6383,7 +6383,7 @@ void __paginginit free_area_init_node(int nid, unsigned long *zones_size,
> free_area_init_core(pgdat);
> }
>
> -#ifdef CONFIG_HAVE_MEMBLOCK
> +#if defined(CONFIG_HAVE_MEMBLOCK) && !defined(CONFIG_FLAT_NODE_MEM_MAP)
> /*
> * Only struct pages that are backed by physical memory are zeroed and
> * initialized by going through __init_single_page(). But, there are some
> @@ -6421,7 +6421,7 @@ void __paginginit zero_resv_unavail(void)
> if (pgcnt)
> pr_info("Reserved but unavailable: %lld pages", pgcnt);
> }
> -#endif /* CONFIG_HAVE_MEMBLOCK */
> +#endif /* CONFIG_HAVE_MEMBLOCK && !CONFIG_FLAT_NODE_MEM_MAP */
>
> #ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
>
> --
> 2.18.0
>

--
Michal Hocko
SUSE Labs