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

[Hao Ge]

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.

Thanks
Best Regards
Hao

> 
> --
> Uladzislau Rezki