Re: [PATCH] mm/sparse: reset section's mem_map when fully deactivated

[Qian Cai <cai@lca.pw>]

> On Jan 15, 2020, at 10:01 PM, Pingfan Liu <kernelfans@gmail.com> wrote:
> 
> When fully deactivated, it is meaningless to keep the value of a section's
> mem_map. And its mem_map will be reassigned during re-added.
> 
> Beside this, it breaks the user space tool "makedumpfile", which makes
> assumption that a hot-removed section having mem_map as NULL.

Well, tools like makedumpfile and the crash utility always has to copy with
low kernel implementation details changes like this. Why is it different this time?

> 
> The bug can be reproduced on IBM POWERVM by "drmgr -c mem -r -q 5" ,
> trigger a crash, and save vmcore by makedumpfile
> 
> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> To: linux-mm@kvack.org
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: kexec@lists.infradead.org
> Cc: Kazuhito Hagio <k-hagio@ab.jp.nec.com>
> ---
> mm/sparse.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 3822ecb..fddac80 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -789,7 +789,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
> 			ms->usage = NULL;
> 		}
> 		memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
> -		ms->section_mem_map = sparse_encode_mem_map(NULL, section_nr);
> +		ms->section_mem_map = NULL;
> 	}
> 
> 	if (section_is_early && memmap)
> -- 
> 2.7.5
> 
>