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

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Message-ID: <20180509210920.GZ32366@dhcp22.suse.cz>
Date:   Wed, 9 May 2018 23:09:20 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, linux-mm@...ck.org,
        mgorman@...hsingularity.net, mingo@...nel.org,
        peterz@...radead.org, rostedt@...dmis.org, fengguang.wu@...el.com,
        dennisszhou@...il.com
Subject: Re: [PATCH] mm: allow deferred page init for vmemmap only

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@wfg-t540p.sh.intel.com
> http://lkml.kernel.org/r/20180419013128.iurzouiqxvcnpbvz@wfg-t540p.sh.intel.com
> http://lkml.kernel.org/r/20180426202619.2768-1-pasha.tatashin@oracle.com

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@...cle.com>
> ---
>  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