Re: [PATCH v11 2/7] mm/vmalloc: undo partial mappings inside the mapping functions

[Hao Ge <hao.ge@linux.dev>]

Hi Suren 

On 2026/10/5 01:42, Suren Baghdasaryan wrote:
> On Wed, Sep 30, 2026 at 12:12 PM Uladzislau Rezki <urezki@gmail.com> wrote:
>>
>> On Tue, Sep 29, 2026 at 04:20:09PM +0800, Hao Ge 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. For example, pcpu_map_pages() and
>>> kmsan_ioremap_page_range() unmap the leftovers themselves, while
>>> vm_module_tags_populate() and the __GFP_NOFAIL retry loop in
>>> __vmalloc_area_node() relied on the mapping functions cleaning up and
>>> did not call anything like vunmap_range() themselves. When the same
>>> range is mapped again, the attempt hits the leftovers and fails, with
>>> BUG() in vmap_pte_range() for huge mappings.
>>>
>>> After discussing with Suren and Ulad, we decided the cleanup belongs
>>> to __vmap_pages_range_noflush() and friends, so the callers no longer
>>> need to unmap the partial mappings themselves. Each function now
>>> undoes the PTEs it installed itself.
>>>
>>> The rollback calls the low-level __vunmap_range_noflush(), it just
>>> clears the PTEs of the range it is given, which is all a rollback
>>> needs. It cannot use vunmap_range_noflush() because these mapping
>>> functions also map the KMSAN shadow and origin, and for a metadata
>>> range its hook would look up the metadata of the metadata, get 0
>>> and BUG() on addr >= end. The failed mappings were never accessed,
>>> no TLB flush needed.
>>>
>>> Fixes: 9376130c390a ("mm/vmalloc: add support for __GFP_NOFAIL")
>>> Fixes: 0f9b685626da ("alloc_tag: populate memory for module tags as needed")
> 
> I don't think this fixes 0f9b685626da. I mean it fixes it but
> indirectly. That patch expects vmap_pages_range() to undo the mappings
> when it fails, which is a reasonable assumption. By fixing
> vmap_pages_xxx we fix all its users, including
> vm_module_tags_populate(). So, it indirectly fixes several callers but
> when adding a Fixes tag we do not list all patches we indirectly fix.
>

Got it, learned something new here. I used to think we had to add a Fixes tag
so stable folks know which branches to backport to.
I went back and re-read Documentation/process/stable-kernel-rules.rst, and turns out
that's not how it works.

Both Fixes tags should be removed then. At the end of the day we're just normalizing the API;
those other commits are merely beneficiaries of this change.

Thanks for your patient guidance.

Best Regards
Hao

>>> Reported-by: Sashiko <sashiko-bot@kernel.org>
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Hao Ge <hao.ge@linux.dev>
>>> ---
>>>  mm/kmsan/shadow.c |  4 ++++
>>>  mm/vmalloc.c      | 31 +++++++++++++++++++++++++++++--
>>>  2 files changed, 33 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/kmsan/shadow.c b/mm/kmsan/shadow.c
>>> index 0c88d89bf0d6..2166086d3dc3 100644
>>> --- a/mm/kmsan/shadow.c
>>> +++ b/mm/kmsan/shadow.c
>>> @@ -258,6 +258,10 @@ int kmsan_vmap_pages_range_noflush(unsigned long start, unsigned long end,
>>>                                           o_pages, page_shift);
>>>       kmsan_leave_runtime();
>>>       if (mapped) {
>>> +             /* Undo the shadow mapping set up above. */
>>> +             kmsan_enter_runtime();
>>> +             __vunmap_range_noflush(shadow_start, shadow_end);
>>> +             kmsan_leave_runtime();
>>>               err = mapped;
>>>               goto ret;
>>>       }
>>>
>> Can we just do that inside the vmalloc? For example in the
>> __vmap_pages_range_noflush() as entry function for all(?) helpers?
>> So we do not need do it manually?
> 
> Agree, that would be better.
> 
>>
>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>> index 859e6d2d57a3..9bbf75706627 100644
>>> --- a/mm/vmalloc.c
>>> +++ b/mm/vmalloc.c
>>> @@ -349,6 +349,10 @@ static int vmap_range_noflush(unsigned long addr, unsigned long end,
>>>       if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
>>>               arch_sync_kernel_mappings(start, end);
>>>
>>> +     /* Undo the PTEs installed before the failure. */
>>> +     if (err)
>>> +             __vunmap_range_noflush(start, end);
>>> +
>>>       return err;
>>>  }
>>>
>>> @@ -363,6 +367,9 @@ 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;
>>>  }
>>>
>>> @@ -667,6 +674,10 @@ static int vmap_small_pages_range_noflush(unsigned long addr, unsigned long end,
>>>       if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
>>>               arch_sync_kernel_mappings(start, end);
>>>
>>> +     /* Undo the PTEs installed before the failure. */
>>> +     if (err)
>>> +             __vunmap_range_noflush(start, end);
>>> +
>>>       return err;
>>>  }
>>>
>> The only one user of that function is __vmap_pages_range_noflush()
>> so we can cover both cases there small and huge path.
>>
>> Is that enough just to do unroll in the below:
>>
>> __vmap_pages_range_noflush()
>> vmap_page_range()
>>
>> functions? It will also fix NOFAIL case:
>>
>> <snip>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 89c327a6ce7d..fa1f357ecf3f 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -359,10 +359,19 @@ int vmap_page_range(unsigned long addr, unsigned long end,
>>
>>         err = vmap_range_noflush(addr, end, phys_addr, pgprot_nx(prot),
>>                                  ioremap_max_page_shift);
>> +       if (err)
>> +               goto error_cleanup_range;
>> +
>>         flush_cache_vmap(addr, end);
>> -       if (!err)
>> -               err = kmsan_ioremap_page_range(addr, end, phys_addr, prot,
>> +       err = kmsan_ioremap_page_range(addr, end, phys_addr, prot,
>>                                                ioremap_max_page_shift);
>> +       if (err)
>> +               goto error_cleanup_range;
>> +
>> +       return 0;
>> +
>> +error_cleanup_range:
>> +       __vunmap_range_noflush(addr, end);
>>         return err;
>>  }
>>
>> @@ -683,26 +692,35 @@ 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;
>>
>> -       WARN_ON(page_shift < PAGE_SHIFT);
>> -
>> -       if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
>> -                       page_shift == PAGE_SHIFT)
>> -               return vmap_small_pages_range_noflush(addr, end, prot, pages);
>> +       if (WARN_ON(addr >= end))
>> +               return -EINVAL;
>>
>> -       for (i = 0; i < nr; i += 1U << (page_shift - PAGE_SHIFT)) {
>> -               int err;
>> +       if (WARN_ON(page_shift < PAGE_SHIFT))
>> +               return -EINVAL;
>>
>> -               err = vmap_range_noflush(addr, addr + (1UL << page_shift),
>> +       if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
>> +               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;
>> +       if (err)
>> +               __vunmap_range_noflush(start, end);
>> +
>> +       /* 0 on success. */
>> +       return err;
>>  }
>>
>>  int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
>> @@ -714,7 +732,13 @@ int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
>>
>>         if (ret)
>>                 return ret;
>> -       return __vmap_pages_range_noflush(addr, end, prot, pages, page_shift);
>> +
>> +       ret = __vmap_pages_range_noflush(addr, end, prot, pages, page_shift);
>> +       if (ret)
>> +               /* Cleanup KMSAN metadata. */
>> +               kmsan_vunmap_range_noflush(addr, end);
>> +
>> +       return ret;
>>  }
>>
>>  static int __vmap_pages_range(unsigned long addr, unsigned long end,
>> <snip>
>>
>> --
>> Uladzislau Rezki