Re: [PATCH v11 4/7] mm: extend the template fast path to zone-device compound tails

[Mike Rapoport <rppt@kernel.org>]

On Mon, Aug 31, 2026 at 07:16:35PM +0800, Li Zhe wrote:
> The template fast path from the previous patch only accelerates head
> pages. Compound tails in memmap_init_compound() still go through the
> normal initialization path one by one.
> 
> Build separate head and tail templates and reuse one prepared tail
> template across the tail pages in a compound range. Head pages preserve
> the existing refcount policy, while compound tails always start with a
> refcount of 0 after prep_compound_tail().
> 
> This extends the template-copy fast path to pfns_per_compound > 1.
> Tail-page PFN-dependent fields are refreshed in the reusable tail
> template before each copy.
> 
> Do not keep a separate non-template fallback for compound tails either.
> These pages are still under memmap initialization, and the
> initialization-time refcount updates are not part of the observable
> lifetime of pages handed out later.
> 
> The impact is controlled for the same reason as for head pages. The
> first tail page still seeds the reusable tail template through the
> normal tail initialization sequence, and the copied tail pages have the
> same final initialized state except for the PFN-dependent fields
> refreshed before each copy.
> 
> Tested in a VM with a 100 GB devdax namespace (align=2097152) on Intel
> Ice Lake server. This test exercises the dax_pmem rebind path and
> measures memmap initialization latency.
> 
> Test procedure:
> Unbind and rebind the dax_pmem driver 30 times, collect memmap
> initialization time from the pr_debug() output of
> memmap_init_zone_device().
> 
> Base(v7.3-rc1):
>   Average of rebinds for dax_pmem driver: 191.20 ms
> 
> With this patch and its prerequisites applied:
>   Average of rebinds for dax_pmem driver: 176.87 ms
> 
> This reduces the average memmap initialization time measured during
> rebind from 191.20 ms to 176.87 ms, or about 7.5%.
> 
> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  mm/mm_init.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)

-- 
Sincerely yours,
Mike.