[PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[Brendan Jackman]

Since the migratetype hygiene patches [0], the locking here is
a bit more formalised, so write it down with an assert.

[0] https://lore.kernel.org/lkml/20240320180429.678181-3-hannes@cmpxchg.org/T/

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 include/linux/memory_hotplug.h | 1 +
 mm/memory_hotplug.c            | 5 +++++
 mm/page_alloc.c                | 4 ++++
 3 files changed, 10 insertions(+)

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index eaac5ae8c05c8ee2f2868d5bc1b04d1f68235b3f..ff9511d9b9e6f775feeb4f897754878ac4c8cdb0 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -175,6 +175,7 @@ void put_online_mems(void);
 
 void mem_hotplug_begin(void);
 void mem_hotplug_done(void);
+bool in_mem_hotplug(void);
 
 /* See kswapd_is_running() */
 static inline void pgdat_kswapd_lock(pg_data_t *pgdat)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 16cf9e17077e359b98a69dc4bca48f4575b9a28c..2b9d1a6267c0872c433fafe8684dd4e2aa821f5f 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -267,6 +267,11 @@ void mem_hotplug_done(void)
 	cpus_read_unlock();
 }
 
+bool in_mem_hotplug(void)
+{
+	return percpu_is_write_locked(&mem_hotplug_lock);
+}
+
 u64 max_mem_size = U64_MAX;
 
 /* add this memory to iomem resource */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 299c9f10bbcd2aa1d35ac6eb5131a2471da825d5..e0516d70835e20e54ff8e43f3aa884c7e36d929a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -417,6 +417,10 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
 
 void set_pageblock_migratetype(struct page *page, int migratetype)
 {
+	lockdep_assert_once(system_state == SYSTEM_BOOTING ||
+		in_mem_hotplug() ||
+		lockdep_is_held(&page_zone(page)->lock));
+
 	if (unlikely(page_group_by_mobility_disabled &&
 		     migratetype < MIGRATE_PCPTYPES))
 		migratetype = MIGRATE_UNMOVABLE;

---
base-commit: d58172d128acbafa2295aa17cc96e28260da9a86
change-id: 20250227-pageblock-lockdep-9628c48d7e08

Best regards,
-- 
Brendan Jackman <jackmanb@google.com>

Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[Andrew Morton]

On Thu, 27 Feb 2025 16:15:47 +0000 Brendan Jackman <jackmanb@google.com> wrote:

> Since the migratetype hygiene patches [0], the locking here is
> a bit more formalised, so write it down with an assert.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -417,6 +417,10 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
>  
>  void set_pageblock_migratetype(struct page *page, int migratetype)
>  {
> +	lockdep_assert_once(system_state == SYSTEM_BOOTING ||
> +		in_mem_hotplug() ||
> +		lockdep_is_held(&page_zone(page)->lock));
> +
>  	if (unlikely(page_group_by_mobility_disabled &&
>  		     migratetype < MIGRATE_PCPTYPES))
>  		migratetype = MIGRATE_UNMOVABLE;
> 

We could add such assertions all over the place.  Why this place in
particular?

Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[Brendan Jackman]

On Thu, Feb 27, 2025 at 02:33:02PM -0800, Andrew Morton wrote:
> On Thu, 27 Feb 2025 16:15:47 +0000 Brendan Jackman <jackmanb@google.com> wrote:
> 
> > Since the migratetype hygiene patches [0], the locking here is
> > a bit more formalised, so write it down with an assert.
> > 
> > ...
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -417,6 +417,10 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
> >  
> >  void set_pageblock_migratetype(struct page *page, int migratetype)
> >  {
> > +	lockdep_assert_once(system_state == SYSTEM_BOOTING ||
> > +		in_mem_hotplug() ||
> > +		lockdep_is_held(&page_zone(page)->lock));
> > +
> >  	if (unlikely(page_group_by_mobility_disabled &&
> >  		     migratetype < MIGRATE_PCPTYPES))
> >  		migratetype = MIGRATE_UNMOVABLE;
> > 
> 
> We could add such assertions all over the place.  Why this place in
> particular?

For the other stuff, it's pretty obvious that it would be protected by
the zone lock (or, I don't know about it!). But it didn't seem totally
self-evident to me that it should protect the pageblock type. So it
seems particularly helpful to have it written in the code.

I may be heavily biased about this though, because of the code I'm
working on for [0]. I use the pageblock type to remember whether it's
mapped in the ASI restricted address space.

[0] https://lore.kernel.org/linux-mm/CA+i-1C1gOBLxRxE5YFGzeayYWBYyE_X6oH4D=9eVePt4=ehTig@mail.gmail.com/T/

Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[Vlastimil Babka]

On 2/28/25 10:20, Brendan Jackman wrote:
> On Thu, Feb 27, 2025 at 02:33:02PM -0800, Andrew Morton wrote:
>> On Thu, 27 Feb 2025 16:15:47 +0000 Brendan Jackman <jackmanb@google.com> wrote:
>> 
>> > Since the migratetype hygiene patches [0], the locking here is
>> > a bit more formalised, so write it down with an assert.
>> > 
>> > ...
>> >
>> > --- a/mm/page_alloc.c
>> > +++ b/mm/page_alloc.c
>> > @@ -417,6 +417,10 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
>> >  
>> >  void set_pageblock_migratetype(struct page *page, int migratetype)
>> >  {
>> > +	lockdep_assert_once(system_state == SYSTEM_BOOTING ||
>> > +		in_mem_hotplug() ||
>> > +		lockdep_is_held(&page_zone(page)->lock));
>> > +
>> >  	if (unlikely(page_group_by_mobility_disabled &&
>> >  		     migratetype < MIGRATE_PCPTYPES))
>> >  		migratetype = MIGRATE_UNMOVABLE;
>> > 
>> 
>> We could add such assertions all over the place.  Why this place in
>> particular?
> 
> For the other stuff, it's pretty obvious that it would be protected by
> the zone lock (or, I don't know about it!). But it didn't seem totally
> self-evident to me that it should protect the pageblock type. So it
> seems particularly helpful to have it written in the code.

I guess that's a good addition to the changelog.

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> 
> I may be heavily biased about this though, because of the code I'm
> working on for [0]. I use the pageblock type to remember whether it's
> mapped in the ASI restricted address space.
> 
> [0] https://lore.kernel.org/linux-mm/CA+i-1C1gOBLxRxE5YFGzeayYWBYyE_X6oH4D=9eVePt4=ehTig@mail.gmail.com/T/
> 

Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[kernel test robot]

Hi Brendan,

kernel test robot noticed the following build errors:

[auto build test ERROR on d58172d128acbafa2295aa17cc96e28260da9a86]

url:    https://github.com/intel-lab-lkp/linux/commits/Brendan-Jackman/mm-page_alloc-Add-lockdep-assertion-for-pageblock-type-change/20250228-002107
base:   d58172d128acbafa2295aa17cc96e28260da9a86
patch link:    https://lore.kernel.org/r/20250227-pageblock-lockdep-v1-1-3701efb331bb%40google.com
patch subject: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change
config: x86_64-buildonly-randconfig-002-20250228 (https://download.01.org/0day-ci/archive/20250301/202503010129.rJvGqZN1-lkp@intel.com/config)
compiler: clang version 19.1.7 (https://github.com/llvm/llvm-project cd708029e0b2869e80abe31ddb175f7c35361f90)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250301/202503010129.rJvGqZN1-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202503010129.rJvGqZN1-lkp@intel.com/

All errors (new ones prefixed by >>):

   In file included from mm/page_alloc.c:19:
   In file included from include/linux/mm.h:2302:
   include/linux/vmstat.h:518:36: warning: arithmetic between different enumeration types ('enum node_stat_item' and 'enum lru_list') [-Wenum-enum-conversion]
     518 |         return node_stat_name(NR_LRU_BASE + lru) + 3; // skip "nr_"
         |                               ~~~~~~~~~~~ ^ ~~~
   In file included from mm/page_alloc.c:44:
   include/linux/mm_inline.h:47:41: warning: arithmetic between different enumeration types ('enum node_stat_item' and 'enum lru_list') [-Wenum-enum-conversion]
      47 |         __mod_lruvec_state(lruvec, NR_LRU_BASE + lru, nr_pages);
         |                                    ~~~~~~~~~~~ ^ ~~~
   include/linux/mm_inline.h:49:22: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum lru_list') [-Wenum-enum-conversion]
      49 |                                 NR_ZONE_LRU_BASE + lru, nr_pages);
         |                                 ~~~~~~~~~~~~~~~~ ^ ~~~
>> mm/page_alloc.c:421:3: error: call to undeclared function 'in_mem_hotplug'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
     421 |                 in_mem_hotplug() ||
         |                 ^
   mm/page_alloc.c:2857:2: warning: arithmetic between different enumeration types ('enum vm_event_item' and 'enum zone_type') [-Wenum-enum-conversion]
    2857 |         __count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   include/linux/vmstat.h:139:34: note: expanded from macro '__count_zid_vm_events'
     139 |         __count_vm_events(item##_NORMAL - ZONE_NORMAL + zid, delta)
         |                           ~~~~~~~~~~~~~ ^ ~~~~~~~~~~~
   mm/page_alloc.c:2974:3: warning: arithmetic between different enumeration types ('enum vm_event_item' and 'enum zone_type') [-Wenum-enum-conversion]
    2974 |                 __count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
         |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   include/linux/vmstat.h:139:34: note: expanded from macro '__count_zid_vm_events'
     139 |         __count_vm_events(item##_NORMAL - ZONE_NORMAL + zid, delta)
         |                           ~~~~~~~~~~~~~ ^ ~~~~~~~~~~~
   mm/page_alloc.c:4745:2: warning: arithmetic between different enumeration types ('enum vm_event_item' and 'enum zone_type') [-Wenum-enum-conversion]
    4745 |         __count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   include/linux/vmstat.h:139:34: note: expanded from macro '__count_zid_vm_events'
     139 |         __count_vm_events(item##_NORMAL - ZONE_NORMAL + zid, delta)
         |                           ~~~~~~~~~~~~~ ^ ~~~~~~~~~~~
   6 warnings and 1 error generated.


vim +/in_mem_hotplug +421 mm/page_alloc.c

   417	
   418	void set_pageblock_migratetype(struct page *page, int migratetype)
   419	{
   420		lockdep_assert_once(system_state == SYSTEM_BOOTING ||
 > 421			in_mem_hotplug() ||
   422			lockdep_is_held(&page_zone(page)->lock));
   423	
   424		if (unlikely(page_group_by_mobility_disabled &&
   425			     migratetype < MIGRATE_PCPTYPES))
   426			migratetype = MIGRATE_UNMOVABLE;
   427	
   428		set_pfnblock_flags_mask(page, (unsigned long)migratetype,
   429					page_to_pfn(page), MIGRATETYPE_MASK);
   430	}
   431	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[Johannes Weiner]

On Sat, Mar 01, 2025 at 01:31:30AM +0800, kernel test robot wrote:
> Hi Brendan,
> 
> kernel test robot noticed the following build errors:
> 
> [auto build test ERROR on d58172d128acbafa2295aa17cc96e28260da9a86]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/Brendan-Jackman/mm-page_alloc-Add-lockdep-assertion-for-pageblock-type-change/20250228-002107
> base:   d58172d128acbafa2295aa17cc96e28260da9a86
> patch link:    https://lore.kernel.org/r/20250227-pageblock-lockdep-v1-1-3701efb331bb%40google.com
> patch subject: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change
> config: x86_64-buildonly-randconfig-002-20250228 (https://download.01.org/0day-ci/archive/20250301/202503010129.rJvGqZN1-lkp@intel.com/config)
> compiler: clang version 19.1.7 (https://github.com/llvm/llvm-project cd708029e0b2869e80abe31ddb175f7c35361f90)
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250301/202503010129.rJvGqZN1-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202503010129.rJvGqZN1-lkp@intel.com/
> 
> All errors (new ones prefixed by >>):
> 
>    In file included from mm/page_alloc.c:19:
>    In file included from include/linux/mm.h:2302:
>    include/linux/vmstat.h:518:36: warning: arithmetic between different enumeration types ('enum node_stat_item' and 'enum lru_list') [-Wenum-enum-conversion]
>      518 |         return node_stat_name(NR_LRU_BASE + lru) + 3; // skip "nr_"
>          |                               ~~~~~~~~~~~ ^ ~~~
>    In file included from mm/page_alloc.c:44:
>    include/linux/mm_inline.h:47:41: warning: arithmetic between different enumeration types ('enum node_stat_item' and 'enum lru_list') [-Wenum-enum-conversion]
>       47 |         __mod_lruvec_state(lruvec, NR_LRU_BASE + lru, nr_pages);
>          |                                    ~~~~~~~~~~~ ^ ~~~
>    include/linux/mm_inline.h:49:22: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum lru_list') [-Wenum-enum-conversion]
>       49 |                                 NR_ZONE_LRU_BASE + lru, nr_pages);
>          |                                 ~~~~~~~~~~~~~~~~ ^ ~~~
> >> mm/page_alloc.c:421:3: error: call to undeclared function 'in_mem_hotplug'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
>      421 |                 in_mem_hotplug() ||

The patch is missing a dummy in_mem_hotplug() in the
!CONFIG_MEMORY_HOTPLUG section of <linux/memory_hotplug.h>.

Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[kernel test robot]

Hi Brendan,

kernel test robot noticed the following build errors:

[auto build test ERROR on d58172d128acbafa2295aa17cc96e28260da9a86]

url:    https://github.com/intel-lab-lkp/linux/commits/Brendan-Jackman/mm-page_alloc-Add-lockdep-assertion-for-pageblock-type-change/20250228-002107
base:   d58172d128acbafa2295aa17cc96e28260da9a86
patch link:    https://lore.kernel.org/r/20250227-pageblock-lockdep-v1-1-3701efb331bb%40google.com
patch subject: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change
config: i386-buildonly-randconfig-005-20250301 (https://download.01.org/0day-ci/archive/20250301/202503010632.CEkonAAQ-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250301/202503010632.CEkonAAQ-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202503010632.CEkonAAQ-lkp@intel.com/

All errors (new ones prefixed by >>):

   In file included from arch/x86/include/asm/bug.h:99,
                    from include/linux/bug.h:5,
                    from include/linux/mmdebug.h:5,
                    from include/linux/mm.h:6,
                    from mm/page_alloc.c:19:
   mm/page_alloc.c: In function 'set_pageblock_migratetype':
>> mm/page_alloc.c:421:17: error: implicit declaration of function 'in_mem_hotplug' [-Werror=implicit-function-declaration]
     421 |                 in_mem_hotplug() ||
         |                 ^~~~~~~~~~~~~~
   include/asm-generic/bug.h:171:32: note: in definition of macro 'WARN_ON'
     171 |         int __ret_warn_on = !!(condition);                              \
         |                                ^~~~~~~~~
   include/linux/lockdep.h:282:14: note: in expansion of macro 'WARN_ON_ONCE'
     282 |         do { WARN_ON_ONCE(debug_locks && !(cond)); } while (0)
         |              ^~~~~~~~~~~~
   mm/page_alloc.c:420:9: note: in expansion of macro 'lockdep_assert_once'
     420 |         lockdep_assert_once(system_state == SYSTEM_BOOTING ||
         |         ^~~~~~~~~~~~~~~~~~~
   cc1: some warnings being treated as errors


vim +/in_mem_hotplug +421 mm/page_alloc.c

   417	
   418	void set_pageblock_migratetype(struct page *page, int migratetype)
   419	{
   420		lockdep_assert_once(system_state == SYSTEM_BOOTING ||
 > 421			in_mem_hotplug() ||
   422			lockdep_is_held(&page_zone(page)->lock));
   423	
   424		if (unlikely(page_group_by_mobility_disabled &&
   425			     migratetype < MIGRATE_PCPTYPES))
   426			migratetype = MIGRATE_UNMOVABLE;
   427	
   428		set_pfnblock_flags_mask(page, (unsigned long)migratetype,
   429					page_to_pfn(page), MIGRATETYPE_MASK);
   430	}
   431	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[Andy Shevchenko]

On Fri, Feb 28, 2025 at 01:28:04PM -0500, Johannes Weiner wrote:
> On Sat, Mar 01, 2025 at 01:31:30AM +0800, kernel test robot wrote:

> The patch is missing a dummy in_mem_hotplug() in the
> !CONFIG_MEMORY_HOTPLUG section of <linux/memory_hotplug.h>.

+1, I just stumbled over and this is not fixed in today's Linux Next. I'm
wondering how this was missed during merge into Linux Next. Stephen?

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[Stephen Rothwell]

[-- Attachment #1: Type: text/plain, Size: 813 bytes --]

Hi Andy,

On Mon, 3 Mar 2025 13:18:42 +0200 Andy Shevchenko <andriy.shevchenko@intel.com> wrote:
>
> On Fri, Feb 28, 2025 at 01:28:04PM -0500, Johannes Weiner wrote:
> > On Sat, Mar 01, 2025 at 01:31:30AM +0800, kernel test robot wrote:  
> 
> > The patch is missing a dummy in_mem_hotplug() in the
> > !CONFIG_MEMORY_HOTPLUG section of <linux/memory_hotplug.h>.  
> 
> +1, I just stumbled over and this is not fixed in today's Linux Next. I'm
> wondering how this was missed during merge into Linux Next. Stephen?

I just get what people put in their trees.  There are no conflicts
around this and none of my builds failed, so I didn't see the problem.
Has someone sent a fix patch to Andrew?  If so, if you forward it to
me, I will add it to linux-next today.

-- 
Cheers,
Stephen Rothwell

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[Brendan Jackman]

On Tue, Mar 04, 2025 at 10:13:57AM +1100, Stephen Rothwell wrote:
> Hi Andy,
> 
> On Mon, 3 Mar 2025 13:18:42 +0200 Andy Shevchenko <andriy.shevchenko@intel.com> wrote:
> >
> > On Fri, Feb 28, 2025 at 01:28:04PM -0500, Johannes Weiner wrote:
> > > On Sat, Mar 01, 2025 at 01:31:30AM +0800, kernel test robot wrote:  
> > 
> > > The patch is missing a dummy in_mem_hotplug() in the
> > > !CONFIG_MEMORY_HOTPLUG section of <linux/memory_hotplug.h>.  
> > 
> > +1, I just stumbled over and this is not fixed in today's Linux Next. I'm
> > wondering how this was missed during merge into Linux Next. Stephen?
> 
> I just get what people put in their trees.  There are no conflicts
> around this and none of my builds failed, so I didn't see the problem.
> Has someone sent a fix patch to Andrew?  If so, if you forward it to
> me, I will add it to linux-next today.

Andrew has backed it out of mm-unstable now. There's a v2 [0] which
still has runtime issues but AFAIK it's not in any tree yet.

[0] https://lore.kernel.org/linux-mm/20250303-pageblock-lockdep-v2-1-3fc0c37e9532@google.com/

In case it helps calibrate expectations: I think this particular patch
had been reviewed but in general some patches get into mm-unstable
without any review being recorded at all. My understanding is that
Andrew squints at it and goes "that looks like it will probably
eventually get merged" and puts it in so that people get a view of
likely upcoming changes.

So if an issue like this reaching linux-next is a big problem then I
think the solution is not to merge mm-unstable. I'm not sure how
high the bar is supposed to be for feeding into linux-next.

Re: [PATCH] mm/page_alloc: Add lockdep assertion for pageblock type change

[David Hildenbrand]

On 04.03.25 11:18, Brendan Jackman wrote:
> On Tue, Mar 04, 2025 at 10:13:57AM +1100, Stephen Rothwell wrote:
>> Hi Andy,
>>
>> On Mon, 3 Mar 2025 13:18:42 +0200 Andy Shevchenko <andriy.shevchenko@intel.com> wrote:
>>>
>>> On Fri, Feb 28, 2025 at 01:28:04PM -0500, Johannes Weiner wrote:
>>>> On Sat, Mar 01, 2025 at 01:31:30AM +0800, kernel test robot wrote:
>>>
>>>> The patch is missing a dummy in_mem_hotplug() in the
>>>> !CONFIG_MEMORY_HOTPLUG section of <linux/memory_hotplug.h>.
>>>
>>> +1, I just stumbled over and this is not fixed in today's Linux Next. I'm
>>> wondering how this was missed during merge into Linux Next. Stephen?
>>
>> I just get what people put in their trees.  There are no conflicts
>> around this and none of my builds failed, so I didn't see the problem.
>> Has someone sent a fix patch to Andrew?  If so, if you forward it to
>> me, I will add it to linux-next today.
> 
> Andrew has backed it out of mm-unstable now. There's a v2 [0] which
> still has runtime issues but AFAIK it's not in any tree yet.
> 
> [0] https://lore.kernel.org/linux-mm/20250303-pageblock-lockdep-v2-1-3fc0c37e9532@google.com/
> 
> In case it helps calibrate expectations: I think this particular patch
> had been reviewed but in general some patches get into mm-unstable
> without any review being recorded at all. My understanding is that
> Andrew squints at it and goes "that looks like it will probably
> eventually get merged" and puts it in so that people get a view of
> likely upcoming changes.
> 
> So if an issue like this reaching linux-next is a big problem then I
> think the solution is not to merge mm-unstable. I'm not sure how
> high the bar is supposed to be for feeding into linux-next.

Last year at LSF/MM I raised that maybe we should have something 
in-between mm-unstable and mm-stable that would get merged into -next. ( 
"mm-almost-stable" / "mm-for-next" ;) )

Alternatively, we could add something like "mm-staging" before 
"mm-unstable" and "mm-stable", whereby only "mm-unstable" would get 
merged into -next.

Ideally, we'd have at least build-bots and some basic sanity checks 
going on on such a staging environment. After a couple of days (not 
more) of survival in such an environment, it could be moved to the tree 
that gets exposed to -next/

I guess the downside is that it's "yet another branch" and "yet more 
build+testing" effort; in particular, if build+testing doesn't happen it 
will be worthless. And likely, a lot of build+testing is happening on 
linux-next only.

I have pretty good results with build bots going crazy on my branches, 
so most stuff that I send (after letting is rest for a couple of days on 
a public branch) doesn't really result in build issues.

-- 
Cheers,

David / dhildenb