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

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

[Hao Ge]

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

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

[Suren Baghdasaryan]

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.

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

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

[Hao Ge]

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
>>