[PATCH] lib/test_hmm: fix NULL deref and wrong direction in devmem fault debug message

[PATCH] lib/test_hmm: fix NULL deref and wrong direction in devmem fault debug message

[liuqiangneo]

From: Qiang Liu <liuqiang@kylinos.cn>

Move pr_debug() inside the `if (dpage)` block to avoid a NULL deref,
and fix the direction label from "sys to dev" to "dev to sys" to match
the device-to-system copy.

Assisted-by: Qoder:Qwen-3.8-MAX-Preview
Signed-off-by: Qiang Liu <liuqiang@kylinos.cn>
---
 lib/test_hmm.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index 6e72a0d8a53d..1dae93ab1e93 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -1151,10 +1151,9 @@ static vm_fault_t dmirror_devmem_fault_alloc_and_copy(struct migrate_vma *args,
 		if (!dpage && !order)
 			return VM_FAULT_OOM;
 
-		pr_debug("migrating from sys to dev pfn src: 0x%lx pfn dst: 0x%lx\n",
-				page_to_pfn(spage), page_to_pfn(dpage));
-
 		if (dpage) {
+			pr_debug("migrating from dev to sys pfn src: 0x%lx pfn dst: 0x%lx\n",
+				 page_to_pfn(spage), page_to_pfn(dpage));
 			lock_page(dpage);
 			*dst |= migrate_pfn(page_to_pfn(dpage));
 		}
-- 
2.43.0


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Re: [PATCH] lib/test_hmm: fix NULL deref and wrong direction in devmem fault debug message

[Andrew Morton]

On Tue, 11 Aug 2026 17:22:55 +0800 liuqiangneo@163.com wrote:

> From: Qiang Liu <liuqiang@kylinos.cn>
> 
> Move pr_debug() inside the `if (dpage)` block to avoid a NULL deref,
> and fix the direction label from "sys to dev" to "dev to sys" to match
> the device-to-system copy.
> 
> ...
>
> --- a/lib/test_hmm.c
> +++ b/lib/test_hmm.c
> @@ -1151,10 +1151,9 @@ static vm_fault_t dmirror_devmem_fault_alloc_and_copy(struct migrate_vma *args,
>  		if (!dpage && !order)
>  			return VM_FAULT_OOM;
>  
> -		pr_debug("migrating from sys to dev pfn src: 0x%lx pfn dst: 0x%lx\n",
> -				page_to_pfn(spage), page_to_pfn(dpage));
> -
>  		if (dpage) {
> +			pr_debug("migrating from dev to sys pfn src: 0x%lx pfn dst: 0x%lx\n",
> +				 page_to_pfn(spage), page_to_pfn(dpage));
>  			lock_page(dpage);
>  			*dst |= migrate_pfn(page_to_pfn(dpage));
>  		}

Current kernel code doesn't look like this?

Re:Re: [PATCH] lib/test_hmm: fix NULL deref and wrong direction in devmem fault debug message

[刘强]

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Hi Andrew,

  Thanks for taking a look.


  Sorry, the previous version was based on mm-everything.
  I've now rebased onto mm-new and resent as v2.
  I compared against
  https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/log/?h=mm-new
  and the code matches the patch context.


  v2:https://lore.kernel.org/all/20260812092856.55296-1-liuqiangneo@163.com/



--

Thanks


Best Regards


Qiang





At 2026-08-12 09:24:40, "Andrew Morton" <akpm@linux-foundation.org> wrote:
>On Tue, 11 Aug 2026 17:22:55 +0800 liuqiangneo@163.com wrote:
>
>> From: Qiang Liu <liuqiang@kylinos.cn>
>> 
>> Move pr_debug() inside the `if (dpage)` block to avoid a NULL deref,
>> and fix the direction label from "sys to dev" to "dev to sys" to match
>> the device-to-system copy.
>> 
>> ...
>>
>> --- a/lib/test_hmm.c
>> +++ b/lib/test_hmm.c
>> @@ -1151,10 +1151,9 @@ static vm_fault_t dmirror_devmem_fault_alloc_and_copy(struct migrate_vma *args,
>>  		if (!dpage && !order)
>>  			return VM_FAULT_OOM;
>>  
>> -		pr_debug("migrating from sys to dev pfn src: 0x%lx pfn dst: 0x%lx\n",
>> -				page_to_pfn(spage), page_to_pfn(dpage));
>> -
>>  		if (dpage) {
>> +			pr_debug("migrating from dev to sys pfn src: 0x%lx pfn dst: 0x%lx\n",
>> +				 page_to_pfn(spage), page_to_pfn(dpage));
>>  			lock_page(dpage);
>>  			*dst |= migrate_pfn(page_to_pfn(dpage));
>>  		}
>
>Current kernel code doesn't look like this?

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