Re: [PATCH v2 09/11] mm/sparse-vmemmap: drop the extra tail page from device DAX reservation

[Muchun Song]

> On Sep 8, 2026, at 17:09, Qi Zheng <qi.zheng@xxxxxxxxx> wrote:
> 
> 
> 
> On 9/8/26 11:03 AM, Muchun Song wrote:
>> The device DAX vmemmap population still reserves one extra tail vmemmap
>> page after the head page.
>> Drop that extra reservation and let the shared tail page cover all tail
>> vmemmap pages after the head page, so DAX follows the same reservation
>> model as HugeTLB.
>> This reduces the reserved vmemmap pages for optimized DAX mappings to
>> one and removes the now-unneeded first-tail population from the generic
>> and powerpc paths to simplify the code as well.
>> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>> ---
>>  arch/powerpc/mm/book3s64/radix_pgtable.c | 46 ++----------------------
>>  include/linux/mm.h                       |  3 +-
>>  mm/mm_init.c                             |  2 +-
>>  mm/sparse-vmemmap.c                      | 13 ++-----
>>  4 files changed, 7 insertions(+), 57 deletions(-)
> 
> Looks like sashiko's response [1] is a false positive.
> 
> [1]. https://sashiko.dev/#/patchset/20260908030335.96549-1-songmuchun%40bytedance.com
> 
> __SetPageReserved() in vmemmap_shared_tail_page() marks the struct page
> entries stored inside the shared vmemmap backing page, i.e. the metadata
> templates that later represent DEV-DAX tail pages. It does not mark the
> struct page of the backing page itself.
> 
> On hot-remove, free_vmemmap_pages() is called with pte_page(*pte), which
> is the backing page. That page is not reserved, so the teardown goes
> through __free_pages(), not free_reserved_pages().
> 
> Right?

Absolutely right.

> 
> If so:
> 
> Acked-by: Qi Zheng <qi.zheng@xxxxxxxxx>

Thanks.