Re: [PATCH v4 1/9] mm/kmsan: undo the shadow mapping when the origin mapping fails

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

Hi Suren

On 2026/11/12 05:03, Suren Baghdasaryan wrote:
> On Tue, Nov 10, 2026 at 11:24 AM Hao Ge <hao.ge@linux.dev> wrote:
>>
>> kmsan_vmap_pages_range_noflush() first maps the shadow pages and then
>> the origin pages. If the second mapping fails, the first one is left
>> mapped, and the callers do not roll it back. The next vmap of the
>> same metadata range hits the stale PTEs again and fails with a
>> WARN_ON() and -EBUSY in vmap_pages_pte_range().
>>
>> Undo the shadow mapping on that error path, the same way
>> kmsan_ioremap_page_range() cleans up after a partial failure.
>> __vunmap_range_noflush() only clears the PTEs; nothing has touched
>> the shadow mapping, so no TLB flush is needed.
>>
>> Fixes: 47ebd0310e89 ("mm: kmsan: handle alloc failures in kmsan_vmap_pages_range_noflush()")
>> Cc: stable@vger.kernel.org
>> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
>> Reviewed-by: Dev Jain <dev.jain@arm.com>
>> Signed-off-by: Hao Ge <hao.ge@linux.dev>
>> ---
>>  mm/kmsan/shadow.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> 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);
> 
> Sorry, there is a problem here. You introduce "can_resched" check to
> avoid calling cond_resched() later in the series (in patch #7), but at
> this point __vunmap_range_noflush() can be called with GFP flags which
> do not allow blocking. Before recent Ulad's change called "kmsan:
> remove hard-coded GFP_KERNEL flags" this function was called only with
> GPF_KERNEL but now that's not the case. I think you can move
> introduction of can_resched flag into this patch.

Agree. I just went back and looked at Ulad's b186a94227b7 ("kmsan:
remove hard-coded GFP_KERNEL flags").

So will do next version. But perhaps it is better to make it a separate
prep patch in the same series? Because besides this call site, the other
places that call __vunmap_range_noflush() need to be changed as well.

Thanks
Best Regards
Hao

> 
>> +               kmsan_leave_runtime();
>>                 err = mapped;
>>                 goto ret;
>>         }
>> --
>> 2.25.1
>>