[PATCH v2 4/7] mm/vmalloc: Extend page table walk to support larger page_shift sizes and eliminate page table rewalk

[Barry Song]

On Wed, May 20, 2026 at 7:53 PM Mike Rapoport <rppt at kernel.org> wrote:
>
> On Thu, 14 May 2026 17:41:05 +0800, Wen Jiang <jiangwenxiaomi at gmail.com> wrote:
>
> Hi,
>
> > vmap_pages_range_noflush_walk() (formerly vmap_small_pages_range_noflush())
> > provides a clean interface by taking struct page **pages and mapping them
> > via direct PTE iteration. This avoids the page table rewalk seen when
> > using vmap_range_noflush() for page_shift values other than PAGE_SHIFT.
> >
> > Extend it to support larger page_shift values, and add PMD- and
> > contiguous-PTE mappings as well. Rename it to vmap_pages_range_noflush_walk()
> > since it now handles more than just small pages.
> >
> > For vmalloc() allocations with VM_ALLOW_HUGE_VMAP, we no longer need to
> > iterate over pages one by one via vmap_range_noflush(), which would
> > otherwise lead to page table rewalk. The code is now unified with the
> > PAGE_SHIFT case by simply calling vmap_pages_range_noflush_walk().
>
> After this patch we have two very simalar page table walkers:
> vmap_pages_range_noflush_walk() and vmap_range_noflush().
>
> The subtly differ at what levels they try huge mappings, how they
> account page table modifucations and, at last vmap_range_noflush() is
> left without support for contiguous mappings.
>
> Is there a fundamental reason to have two page walkers?
> Is there a reason not to support contiguous mappings in
> vmap_range_noflush()?

Hi Mike,

They are two completely different cases. In one case, you have a contiguous
physical address range for ioremap(). Since the range is fully contiguous,
many things can be simplified. We already support large mappings there, but
this patchset still improves performance through larger batching.

In the other case, you have a pages[] array whose pages are not physically
contiguous as a whole. You iterate over the pages and attempt to create large
mappings whenever a subset of adjacent pages happens to be physically
contiguous.

Best Regards
Barry