Re: [PATCH v5] mm: don't allow deferred pages with NEED_PER_CPU_KM

[Michal Hocko]

On Tue 15-05-18 13:51:24, Pavel Tatashin wrote:
> It is unsafe to do virtual to physical translations before mm_init() is
> called if struct page is needed in order to determine the memory section
> number (see SECTION_IN_PAGE_FLAGS). This is because only in mm_init() we
> initialize struct pages for all the allocated memory when deferred struct
> pages are used.
> 
> My recent fix exposed this problem, because it greatly reduced number of
> pages that are initialized before mm_init(), but the problem existed even
> before my fix, as Fengguang Wu found.
> 
> Below is a more detailed explanation of the problem.
> 
> We initialize struct pages in four places:
> 
> 1. Early in boot a small set of struct pages is initialized to fill
> the first section, and lower zones.
> 2. During mm_init() we initialize "struct pages" for all the memory
> that is allocated, i.e reserved in memblock.
> 3. Using on-demand logic when pages are allocated after mm_init call (when
> memblock is finished)
> 4. After smp_init() when the rest free deferred pages are initialized.
> 
> The problem occurs if we try to do va to phys translation of a memory
> between steps 1 and 2. Because we have not yet initialized struct pages for
> all the reserved pages, it is inherently unsafe to do va to phys if the
> translation itself requires access of "struct page" as in case of this
> combination: CONFIG_SPARSE && !CONFIG_SPARSE_VMEMMAP
> 
> The following path exposes the problem:
> 
> start_kernel()
>  trap_init()
>   setup_cpu_entry_areas()
>    setup_cpu_entry_area(cpu)
>     get_cpu_gdt_paddr(cpu)
>      per_cpu_ptr_to_phys(addr)
>       pcpu_addr_to_page(addr)
>        virt_to_page(addr)
>         pfn_to_page(__pa(addr) >> PAGE_SHIFT)
> 
> We disable this path by not allowing NEED_PER_CPU_KM with deferred struct
> pages feature.
> 
> The problems are discussed in these threads:
> http://lkml.kernel.org/r/20180418135300.inazvpxjxowogyge@xxxxxxxxxxxxxxxxxxxxxx
> http://lkml.kernel.org/r/20180419013128.iurzouiqxvcnpbvz@xxxxxxxxxxxxxxxxxxxxxx
> http://lkml.kernel.org/r/20180426202619.2768-1-pasha.tatashin@xxxxxxxxxx
> 
> Fixes: 3a80a7fa7989 ("mm: meminit: initialise a subset of struct pages if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set")
> Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks a lot!

> ---
>  mm/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index d5004d82a1d6..e14c01513bfd 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -636,6 +636,7 @@ config DEFERRED_STRUCT_PAGE_INIT
>  	default n
>  	depends on NO_BOOTMEM
>  	depends on !FLATMEM
> +	depends on !NEED_PER_CPU_KM
>  	help
>  	  Ordinarily all struct pages are initialised during early boot in a
>  	  single thread. On very large machines this can take a considerable
> -- 
> 2.17.0

-- 
Michal Hocko
SUSE Labs