Re: [PATCHv2] mm/sparse: reset section's mem_map when fully deactivated

[Michal Hocko <mhocko@suse.com>]

On Mon 20-01-20 10:33:14, Pingfan Liu wrote:
> After commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug"),
> when a mem section is fully deactivated, section_mem_map still records the
> section's start pfn, which is not used any more and will be reassigned
> during re-added.
> 
> In analogy with alloc/free pattern, it is better to clear all fields of
> section_mem_map.
> 
> Beside this, it breaks the user space tool "makedumpfile" [1], which makes
> assumption that a hot-removed section has mem_map as NULL, instead of
> checking directly against SECTION_MARKED_PRESENT bit. (makedumpfile will be
> better to change the assumption, and need a patch)
> 
> The bug can be reproduced on IBM POWERVM by "drmgr -c mem -r -q 5" ,
> trigger a crash, and save vmcore by makedumpfile

While makedumpfile lives very closely to the kernel and occasional
breakage is to be expected I still believe that Fixes: ba72b4c8cf60
is due.

> [1]: makedumpfile, commit e73016540293 ("[v1.6.7] Update version")
> 
> 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: Baoquan He <bhe@redhat.com>
> Cc: Qian Cai <cai@lca.pw>
> Cc: kexec@lists.infradead.org
> Cc: Kazuhito Hagio <k-hagio@ab.jp.nec.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> v1 -> v2:
>  make an explicit convertion from NULL to ulong
>  improve commit log
>  
>  mm/sparse.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 3822ecb..3918fc3 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 = (unsigned long)NULL;
>  	}
>  
>  	if (section_is_early && memmap)
> -- 
> 2.7.5

-- 
Michal Hocko
SUSE Labs