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

[David Hildenbrand (Arm)]

On 4/1/26 04:41, Muchun Song wrote:
> 
> 
>> On Apr 1, 2026, at 04:42, David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
>>
>> On 3/31/26 13:37, 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@xxxxxxxxxxxxx>
>>> ---
>>> 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;
>>
>>
>> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> 
> Thanks.
> 
>>
>> I have some cleanup patches lying around that cleanup that code heavily.
>> I think I get rid of this questionable "failed to allocate" case entirely.
> 
> It's truly a coincidence — I also have a piece of code locally
> that does something similar. Since allocation failure would also
> affect subsequent startup processes, I simply made it panic when
> allocation fails.

Don't use BUG_ON, use actual panic(). :)

And yes, we should rip out that handling.

-- 
Cheers,

David