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

[Muchun Song <muchun.song@linux.dev>]

> On Apr 1, 2026, at 04:10, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> On Tue, 31 Mar 2026 19:37:24 +0800 Muchun Song <songmuchun@bytedance.com> 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.
> 
> Here I go again ;)  Are there userspace impacts?

Those pre-inited sections (HugeTLB pages) are not activated. However, such
failures are extremely rare, so I don't see any major issues.

> 
> AI review thinks it found a different bug:
> https://sashiko.dev/#/patchset/20260331113724.2080833-1-songmuchun@bytedance.com

I don't think the issue reported by AI is a real problem, because the
allocation of sparse_usagebuf has already taken these hugetlb sections
into account.

Thanks.