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 05:49:50 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Baoquan He <bhe@...hat.com>
Cc:     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 02/01/2018 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?