Re: [PATCH] mm/sparse: fix preinited section_mem_map clobbering on failure path

Muchun Song
2026-04-01T02:29:28Z

> On Apr 1, 2026, at 02:34, Donet Tom <donettom@linux.ibm.com> wrote:
>
> Hi Muchun

Hi,

>
> On 3/31/26 5:07 PM, Muchun Song wrote:
>> sparse_init_nid() is careful to leave alone every section whose vmemmap
>> has already been set up by sparse_vmemmap_init_nid_early(); it only
>> clears section_mem_map for the rest:
>>
>> if (!preinited_vmemmap_section(ms))
>> ms->section_mem_map = 0;
>>
>> A leftover line after that conditional block
>>
>> ms->section_mem_map = 0;
>>
>> was supposed to be deleted but was missed in the failure path, causing the
>> field to be overwritten for all sections when memory allocation fails,
>> effectively destroying the pre-initialization check.
>>
>> Drop the stray assignment so that preinited sections retain their
>> already valid state.
>>
>> Fixes: d65917c42373 ("mm/sparse: allow for alternate vmemmap section init at boot")
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>> mm/sparse.c | 1 -
>> 1 file changed, 1 deletion(-)
>>
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index c2eb36bfb86d..3a14b733bf71 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -584,7 +584,6 @@ static void __init sparse_init_nid(int nid, unsigned long pnum_begin,
>> ms = __nr_to_section(pnum);
>> if (!preinited_vmemmap_section(ms))
>> ms->section_mem_map = 0;
>> - ms->section_mem_map = 0;
>
>
> This looks correct to me.
>
> I have a couple of questions:
>
> 1. As I understand, section_mem_map initially stores the nid
> during early boot, and later it stores a pointer to an
> array of struct page. In sparse_init_nid(), the struct page
> array is stored in section_mem_map via
> sparse_init_early_section(). If
> __populate_section_memmap() fails, we are clearing the nid
> stored in section_mem_map right?

Right.

>
> 2. Another question: if sparse_init_nid() fails for some
> sections, there is no retry mechanism to add them again,
> correct?

Right.

>
> 3. when ms->section_mem_map is set to 0
> for a pre-initialized section, does it only affect the
> pre-initialization check, or could it lead to other issues?
>

Only affect pre-initialization. No other issues.

Thanks.

>
> - Donet
>
>> }
>> }