Re: [PATCH 5/9] mm/rmap: batch unmap folios belonging to uffd-wp VMAs

[Barry Song <21cnbao@gmail.com>]

On Wed, Mar 11, 2026 at 7:32 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Tue, Mar 10, 2026 at 4:34 PM Lorenzo Stoakes (Oracle) <ljs@kernel.org> wrote:
> >
> > On Tue, Mar 10, 2026 at 01:00:09PM +0530, Dev Jain wrote:
> > > The ptes are all the same w.r.t belonging to the same type of VMA, and
> > > being marked with uffd-wp or all being not marked. Therefore we can batch
> > > set uffd-wp markers through install_uffd_wp_ptes_if_needed, and enable
> > > batched unmapping of folios belonging to uffd-wp VMAs by dropping that
> > > condition from folio_unmap_pte_batch.
> > >
> > > It may happen that we don't batch over the entire folio in one go, in which
> > > case, we must skip over the current batch. Add a helper to do that -
> > > page_vma_mapped_walk_jump() will increment the relevant fields of pvmw
> > > by nr pages.
> > >
> > > I think that we can get away with just incrementing pvmw->pte
> > > and pvmw->address, since looking at the code in page_vma_mapped.c,
> > > pvmw->pfn and pvmw->nr_pages are used in conjunction, and pvmw->pgoff
> > > and pvmw->nr_pages (in vma_address_end()) are used in conjunction,
> > > cancelling out the increment and decrement in the respective fields. But
> > > let us not rely on the pvmw implementation and keep this simple.
> >
> > This isn't simple...
> >
> > >
> > > Export this function to rmap.h to enable future reuse.
> > >
> > > Signed-off-by: Dev Jain <dev.jain@arm.com>
> > > ---
> > >  include/linux/rmap.h | 10 ++++++++++
> > >  mm/rmap.c            |  8 +++-----
> > >  2 files changed, 13 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > > index 8dc0871e5f001..1b7720c66ac87 100644
> > > --- a/include/linux/rmap.h
> > > +++ b/include/linux/rmap.h
> > > @@ -892,6 +892,16 @@ static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
> > >               spin_unlock(pvmw->ptl);
> > >  }
> > >
> > > +static inline void page_vma_mapped_walk_jump(struct page_vma_mapped_walk *pvmw,
> > > +             unsigned int nr)
> >
> > unsigned long nr_pages... 'nr' is meaningless and you're mixing + matching types
> > for no reason.
> >
> > > +{
> > > +     pvmw->pfn += nr;
> > > +     pvmw->nr_pages -= nr;
> > > +     pvmw->pgoff += nr;
> > > +     pvmw->pte += nr;
> > > +     pvmw->address += nr * PAGE_SIZE;
> > > +}
> >
> > I absolutely hate this. It's extremely confusing, especially since you're now
> > going from looking at 1 page to nr_pages - 1, jump doesn't really mean anything
> > here, you're losing sight of the batch size and exposing a silly detail to the
> > caller, and I really don't want to 'export' this at this time.
>
> I’m fairly sure I raised the same concern when Dev first suggested this,
> but somehow it seems my comment was completely overlooked. :-)
>
> >
> > If we must have this, can you please make it static in rmap.c at least for the
> > time being.
> >
> > Or perhaps instead, have a batched variant of page_vma_mapped_walk(), like
> > page_vma_mapped_walk_batch()?
>
> Right now, for non-anon pages we face the same issues, but
> page_vma_mapped_walk() can skip those PTEs once it finds that
> nr - 1 PTEs are none.
>
> next_pte:
>                 do {
>                         pvmw->address += PAGE_SIZE;
>                         if (pvmw->address >= end)
>                                 return not_found(pvmw);
>                         /* Did we cross page table boundary? */
>                         if ((pvmw->address & (PMD_SIZE - PAGE_SIZE)) == 0) {
>                                 if (pvmw->ptl) {
>                                         spin_unlock(pvmw->ptl);
>                                         pvmw->ptl = NULL;
>                                 }
>                                 pte_unmap(pvmw->pte);
>                                 pvmw->pte = NULL;
>                                 pvmw->flags |= PVMW_PGTABLE_CROSSED;
>                                 goto restart;
>                         }
>                         pvmw->pte++;
>                 } while (pte_none(ptep_get(pvmw->pte)));
>
> The difference now is that swap entries cannot be skipped.
>
> If we're trying to find `page_vma_mapped_walk_batch()`, I suppose
> it could be like this?
>
> bool page_vma_mapped_walk_batch(struct page_vma_mapped_walk *pvmw,
> unsigned long nr)
> {
> ...
> }
>
> static inline bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> {
>         return page_vma_mapped_walk_batch(pvmw, 1);
> }

Another approach might be to introduce a flag so that
page_vma_mapped_walk() knows we are doing batched unmaps
and can skip nr - 1 swap entries.

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 8dc0871e5f00..bf03ae006366 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -856,6 +856,9 @@ struct page *make_device_exclusive(struct
mm_struct *mm, unsigned long addr,
 /* Look for migration entries rather than present PTEs */
 #define PVMW_MIGRATION         (1 << 1)

+/* Batched unmap: skip swap entries. */
+#define PVMW_BATCH_UNMAP       (1 << 2)
+
 /* Result flags */

 /* The page is mapped across page table boundary */


Thanks
Barry