Re: [akpm-mm:mm-new] BUILD SUCCESS WITH UNVERIFIED WARNING 34cacd0ac7c10dc9e34e1b9f28a2b78d335cb487

Thanks.  A few head-scratchers for hmm people, please:

On Sat, 25 Jul 2026 15:11:38 +0800 kernel test robot <[email protected]> wrote:

> tree/branch: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-new
> branch HEAD: 34cacd0ac7c10dc9e34e1b9f28a2b78d335cb487  mm/swap, PM: hibernate: atomically replace hibernation pin
> 
> Unverified Warning (likely false positive, kindly check if interested):
> 
>     https://lore.kernel.org/oe-kbuild/[email protected]
>     https://lore.kernel.org/oe-kbuild/[email protected]
> 
>     mm/hmm.c:673 hmm_do_fault() error: we previously assumed 'hmm_vma_walk->locked' could be null (see line 654)
>     mm/vmpressure.c:123 vmpressure() warn: check likely/unlikely parentheses
> 
> Warning ids grouped by kconfigs:
> 
> recent_errors
> |-- microblaze-randconfig-r072-20260725
> |   `-- mm-hmm.c-hmm_do_fault()-error:we-previously-assumed-hmm_vma_walk-locked-could-be-null-(see-line-)

With this microblaze config on current mainline I hit

mm/hmm.c:804:25: error: implicit declaration of function 'mmu_interval_read_begin'; did you mean 'mmu_interval_check_retry'? [-Wimplicit-function-declaration]
  804 |                         mmu_interval_read_begin(range->notifier);

And after adding the hmm changes from mm-unstable I also hit

mm/hmm.c:805:25: error: implicit declaration of function 'mmu_interval_read_begin'; did you mean 'mmu_interval_check_retry'? [-Wimplicit-function-declaration]
  805 |                         mmu_interval_read_begin(range->notifier);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~
      |                         mmu_interval_check_retry

Both of which can be fixed with

--- a/include/linux/mmu_notifier.h~a
+++ a/include/linux/mmu_notifier.h
@@ -576,6 +576,19 @@ static inline void _mmu_notifier_range_i
 	_mmu_notifier_range_init(range, start, end)
 
 static inline bool
+mmu_interval_check_retry(struct mmu_interval_notifier *interval_sub,
+			 unsigned long seq)
+{
+	return true;
+}
+
+static inline unsigned long
+mmu_interval_read_begin(struct mmu_interval_notifier *interval_sub)
+{
+	return 0;
+}
+
+static inline bool
 mmu_notifier_range_blockable(const struct mmu_notifier_range *range)
 {
 	return true;

But I do wonder whether we should be compiling hmm at all if
CONFIG_MMU_NOTIFIER=n?


And your hmm_do_fault() warning seems legit:

	if (hmm_vma_walk->locked)
		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;

...
			*hmm_vma_walk->locked = false;

and the blame seems to lie with Stanislav's "mm/hmm: add
hmm_range_fault_unlocked_timeout() for mmap lock-drop support".