Re: [PATCH v3 1/6] mm/sparse-vmemmap: drop VMEMMAP_POPULATE_DAX

On Tue, Sep 29, 2026 at 01:32:26PM +0800, Muchun Song wrote:

VMEMMAP_POPULATE_DAX currently distinguishes DAX vmemmap population in
two places: it keeps allocations on the normal path and takes a reference
when a backing page is supplied for reuse.

After Device DAX switched to the common per-zone shared tail page, both
conditions can be determined locally. DAX supplies ptpfn for every shared
tail mapping and requests an allocation only for compound head mappings,
whose PFNs are not optimizable. Therefore, vmemmap_optimizable_pfn() alone
selects the correct allocation path.

When ptpfn is supplied, the caller is reusing an existing backing page.
Once the slab allocator is available, take a reference for each reused
mapping to balance the release performed by vmemmap_free(). Although the
buddy allocator is available before slab, no vmemmap population occurs in
that interval. Earlier mappings are backed by memblock/reserved memory and
do not need page reference accounting.

Remove VMEMMAP_POPULATE_DAX and the flags argument from the vmemmap
population helpers.

Signed-off-by: Muchun Song <redacted>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
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LGTM! Feel free to add:

Reviewed-by: Lance Yang <lance.yang@linux.dev>