linux-kernel - Re: [PATCH 2/2] mm/sparse.c: Add nr_present_sections to change the mem

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Date:   Thu, 1 Feb 2018 22:19:34 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        mhocko@...e.com, tglx@...utronix.de, douly.fnst@...fujitsu.com
Subject: Re: [PATCH 2/2] mm/sparse.c: Add nr_present_sections to change the
 mem_map allocation

On 02/01/18 at 05:49am, Dave Hansen wrote:
> On 02/01/2018 02:16 AM, Kirill A. Shutemov wrote:
> > On Thu, Feb 01, 2018 at 03:19:56PM +0800, Baoquan He wrote:
> >> In sparse_init(), we allocate usemap_map and map_map which are pointer
> >> array with the size of NR_MEM_SECTIONS. The memory consumption can be
> >> ignorable in 4-level paging mode. While in 5-level paging, this costs
> >> much memory, 512M. Kdump kernel even can't boot up with a normal
> >> 'crashkernel=' setting.
> >>
> >> Here add a new variable to record the number of present sections. Let's
> >> allocate the usemap_map and map_map with the size of nr_present_sections.
> >> We only need to make sure that for the ith present section, usemap_map[i]
> >> and map_map[i] store its usemap and mem_map separately.
> >>
> >> This change can save much memory on most of systems. Anytime, we should
> >> avoid to define array or allocate memory with the size of NR_MEM_SECTIONS.
> > That's very desirable outcome. But I don't know much about sparsemem.
> 
> ... with the downside being that we can no longer hot-add memory that
> was not part of the original, present sections.
> 
> Is that OK?

Thanks for looking into this, Dave!

I suppose these functions changed here are only called during system
bootup, namely in paging_init(). Hot-add memory goes in a different
path, __add_section() -> sparse_add_one_section(), different called
functions.