Re: [PATCH] mm: allow deferred page init for vmemmap only

From: Michal Hocko
Date: Wed May 09 2018 - 17:09:27 EST


On Wed 09-05-18 15:17:13, 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.
>
> Since FLATMEM is already disallowed for deferred struct pages, it makes
> sense to allow deferred struct pages only on systems with
> SPARSEMEM_VMEMMAP.

This doesn't really explain why CONFIG_SPARSMEM or DISCONTIG has the
problem.

> 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

Using references is good but there should be a highlevel summary in the
chaneglog as well.

> 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>
> ---
> mm/Kconfig | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index d5004d82a1d6..1cd32d67ca30 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -635,7 +635,7 @@ config DEFERRED_STRUCT_PAGE_INIT
> bool "Defer initialisation of struct pages to kthreads"
> default n
> depends on NO_BOOTMEM
> - depends on !FLATMEM
> + depends on SPARSEMEM_VMEMMAP
> 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