Re: [RFC PATCH 2/4] mm/vmalloc: undo partial mappings inside the mapping functions

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Oct 13, 2026 at 03:23:14PM +0400, Suren Baghdasaryan wrote:
> On Fri, Oct 9, 2026 at 10:36 AM Hao Ge <hao.ge@linux.dev> wrote:
> >
> > __vmap_pages_range_noflush() and friends can install some PTEs before
> > failing and leave them mapped, and the kernel callers do not agree on
> > who cleans them up. pcpu_map_pages() and kmsan_ioremap_page_range()
> > roll back what they mapped before the failure, while
> > vm_module_tags_populate() and the __GFP_NOFAIL retry loop in
> > __vmalloc_area_node() rely on the mapping functions cleaning up and
> > do not call anything like vunmap_range() themselves. When the same
> > range is mapped again, the attempt hits the leftovers and fails, with
> > a BUG() in vmap_pte_range() for huge mappings and a warning on the
> > small-page path.
> >
> > After discussing with Suren and Ulad, we decided to put the rollback
> > into the entry points of the vmap API rather than into every low-level
> > helper. __vmap_pages_range_noflush() undoes the whole range it was
> > asked to map when it fails, and vmap_page_range() undoes its range
> > for the ioremap-style mappings.
> >
> > The rollback is __vunmap_range_noflush(), it only clears the PTEs,
> > no TLB flush, nothing has touched these mappings.
> > vmap_pages_range_noflush() drops the KMSAN metadata when the data
> > mapping fails, and an empty range bails out with -EINVAL now, which
> > used to BUG_ON() on the small-page path and map nothing at all on
> > the huge-page one.
> >
> > Reported-by: Sashiko <sashiko-bot@kernel.org>
> > Suggested-by: Uladyslau Rezki <urezki@gmail.com>
> > Signed-off-by: Hao Ge <hao.ge@linux.dev>
> > ---
> >  mm/vmalloc.c | 44 ++++++++++++++++++++++++++++++++------------
> >  1 file changed, 32 insertions(+), 12 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index db669103dc66..e1b376f57dd2 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -363,6 +363,9 @@ int vmap_page_range(unsigned long addr, unsigned long end,
> >         if (!err)
> >                 err = kmsan_ioremap_page_range(addr, end, phys_addr, prot,
> >                                                ioremap_max_page_shift);
> > +       if (err)
> > +               __vunmap_range_noflush(addr, end);
> > +
> >         return err;
> >  }
> >
> > @@ -683,27 +686,35 @@ int __vmap_pages_range_noflush(unsigned long addr, unsigned long end,
> >                 pgprot_t prot, struct page **pages, unsigned int page_shift)
> >  {
> >         unsigned int i, nr = (end - addr) >> PAGE_SHIFT;
> > +       unsigned long start = addr;
> > +       int err = 0;
> > +
> > +       if (WARN_ON_ONCE(addr >= end))
> > +               return -EINVAL;
> >
> >         if (WARN_ON_ONCE(page_shift < PAGE_SHIFT))
> >                 return -EINVAL;
> >
> >         if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
> > -                       page_shift == PAGE_SHIFT)
> > -               return vmap_small_pages_range_noflush(addr, end, prot, pages);
> > -
> > -       for (i = 0; i < nr; i += 1U << (page_shift - PAGE_SHIFT)) {
> > -               int err;
> > -
> > -               err = vmap_range_noflush(addr, addr + (1UL << page_shift),
> > +                       page_shift == PAGE_SHIFT) {
> > +               err = vmap_small_pages_range_noflush(addr, end, prot, pages);
> > +       } else {
> > +               for (i = 0; i < nr; i += 1U << (page_shift - PAGE_SHIFT)) {
> > +                       err = vmap_range_noflush(addr, addr + (1UL << page_shift),
> >                                         page_to_phys(pages[i]), prot,
> >                                         page_shift);
> > -               if (err)
> > -                       return err;
> > +                       if (err)
> > +                               break;
> >
> > -               addr += 1UL << page_shift;
> > +                       addr += 1UL << page_shift;
> > +               }
> >         }
> >
> > -       return 0;
> > +       /* Undo the PTEs installed before the failure. */
> > +       if (err)
> > +               __vunmap_range_noflush(start, end);
> 
> Would it be too complicated to track how much was actually mapped and
> unmap only that portion?
> 
I think it is possible to vunmap the range which is already unmapped.
i.e. in this case it is better to no to track anything as soon as a
mapping failure is a corner case. Also it is more reliable without
extra track logic.

--
Uladzislau Rezki