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

[Suren Baghdasaryan]

On Wed, Oct 14, 2026 at 11:28 AM Hao Ge <hao.ge@xxxxxxxxx> wrote:
>
> Hi Uladzislau and Suren
>
> On 2026/10/14 00:39, Uladzislau Rezki wrote:
> > 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@xxxxxxxxx> 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@xxxxxxxxxx>
> >>> Suggested-by: Uladyslau Rezki <urezki@xxxxxxxxx>
> >>> Signed-off-by: Hao Ge <hao.ge@xxxxxxxxx>
> >>> ---
> >>>  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.
>
> Thank you both.
> I take note of Suren's concerns. The worst case is a single PMD where only
> the first page was installed, __vunmap_range_noflush() then loops 512 times
> to undo one PTE, plus 511 empty ones.
>
> And tracking the exact progress would require vmap_pages_p4d/pud/pmd_range()
> to report how far they got. That also puts counting instructions on the successful
> mapping path, while the saving only pays off on the failure one.
>
> So I agree with Ulad.

Got it. Makes sense.

>
> Thanks
> Best Regards
> Hao
>
> >
> > --
> > Uladzislau Rezki