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

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 26 Jul 2026 11:41:26 -0700 Stanislav Kinsburskii <skinsburskii@gmail.com> wrote:

> > But I do wonder whether we should be compiling hmm at all if
> > CONFIG_MMU_NOTIFIER=n?
> > 
> 
> Documentation/mm/hmm.rst explicitly states:
> 
> "Address space mirroring's main objective is to allow duplication of a range of
> CPU page table into a device page table; HMM helps keep both synchronized. A
> device driver that wants to mirror a process address space must start with the
> registration of a mmu_interval_notifier"
> 
> I think making CONFIG_HMM_MIRROR dependent on the CONFIG_MMU_NOTIFIER is the right thing to do.

OK, thanks.  How about we use select.  select kinda sucks but we're
doing `select MMU_NOTIFIER' all over the place already.


From: Andrew Morton <akpm@linux-foundation.org>
Subject: mm/hmm: make CONFIG_HMM_MIRROR select CONFIG_MMU_NOTIFIER
Date: Mon Jul 27 05:31:14 PM PDT 2026

hmm doesn't currently build well with MMU_NOTIFIER=n:

With microblaze
https://download.01.org/0day-ci/archive/20260702/202607021433.DYT5fDqE-lkp@intel.com/config
(CONFIG_MMU_NOTIFIER=n):

mm/hmm.c: In function 'hmm_range_fault_unlocked_timeout':
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);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~
      |                         mmu_interval_check_retry

Quoting Stanislav:

:  Documentation/mm/hmm.rst explicitly states:
: 
:  "Address space mirroring's main objective is to allow duplication of a
:  range of CPU page table into a device page table; HMM helps keep both
:  synchronized.  A device driver that wants to mirror a process address
:  space must start with the registration of a mmu_interval_notifier"
: 
:  I think making CONFIG_HMM_MIRROR dependent on the CONFIG_MMU_NOTIFIER
:  is the right thing to do.

So select MMU_NOTIFIER if hmm.c is to be compiled.

Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Stanislav Kinsburskii <skinsburskii@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/Kconfig |    1 +
 1 file changed, 1 insertion(+)

--- a/mm/Kconfig~mm-hmm-make-config_hmm_mirror-select-config_mmu_notifier
+++ a/mm/Kconfig
@@ -1250,6 +1250,7 @@ config ZONE_DEVICE
 config HMM_MIRROR
 	bool
 	depends on MMU
+	select MMU_NOTIFIER
 
 config GET_FREE_REGION
 	bool
_