Re: [Patch mm-hotfixes v5] mm/page_vma_mapped: fix device-private PMD handling

[Lance Yang <lance.yang@linux.dev>]

On Wed, Jul 01, 2026 at 05:36:33PM +0200, David Hildenbrand (Arm) wrote:
>On 7/1/26 16:33, Klara Modin wrote:
>> Hi,

Hi,

[...]
>> 
>> This results in a build bug for my Raspberry Pi 1:

Thanks for reporting this!

>>  In file included from <command-line>:
>>  In function ‘check_pmd’,
>>      inlined from ‘page_vma_mapped_walk’ at /home/klara/git/linux/trees/bisect/mm/page_vma_mapped.c:256:10:
>>  /home/klara/git/linux/trees/bisect/include/linux/compiler_types.h:702:45: error: call to ‘__compiletime_assert_433’ declared with attribute error: BUILD_BUG failed
>>    702 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>>        |                                             ^
>>  /home/klara/git/linux/trees/bisect/include/linux/compiler_types.h:683:25: note: in definition of macro ‘__compiletime_assert’
>>    683 |                         prefix ## suffix();                             \
>>        |                         ^~~~~~
>>  /home/klara/git/linux/trees/bisect/include/linux/compiler_types.h:702:9: note: in expansion of macro ‘_compiletime_assert’
>>    702 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>>        |         ^~~~~~~~~~~~~~~~~~~
>>  /home/klara/git/linux/trees/bisect/include/linux/build_bug.h:40:37: note: in expansion of macro ‘compiletime_assert’
>>     40 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
>>        |                                     ^~~~~~~~~~~~~~~~~~
>>  /home/klara/git/linux/trees/bisect/include/linux/build_bug.h:60:21: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
>>     60 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
>>        |                     ^~~~~~~~~~~~~~~~
>>  /home/klara/git/linux/trees/bisect/include/linux/huge_mm.h:113:28: note: in expansion of macro ‘BUILD_BUG’
>>    113 | #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
>>        |                            ^~~~~~~~~
>>  /home/klara/git/linux/trees/bisect/include/linux/huge_mm.h:117:26: note: in expansion of macro ‘HPAGE_PMD_SHIFT’
>>    117 | #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
>>        |                          ^~~~~~~~~~~~~~~
>>  /home/klara/git/linux/trees/bisect/include/linux/huge_mm.h:118:26: note: in expansion of macro ‘HPAGE_PMD_ORDER’
>>    118 | #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
>>        |                          ^~~~~~~~~~~~~~~
>>  /home/klara/git/linux/trees/bisect/mm/page_vma_mapped.c:142:20: note: in expansion of macro ‘HPAGE_PMD_NR’
>>    142 |         if ((pfn + HPAGE_PMD_NR - 1) < pvmw->pfn)
>>        |                    ^~~~~~~~~~~~
>> 
>> bisect log:
>> 
>>  # bad: [be5c93fa674f0fc3c8f359c2143abce6bbb422e6] Add linux-next specific files for 20260630
>>  git bisect start 'HEAD'
>>  # status: waiting for 'good' commit(s), 'bad' commit known
>>  # good: [dc59e4fea9d83f03bad6bddf3fa2e52491777482] Linux 7.2-rc1
>>  git bisect good dc59e4fea9d83f03bad6bddf3fa2e52491777482
>>  # bad: [6148219e90732fd06f5d7a498bda974e6a43ab4b] Merge branch 'nand/next' of https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git
>>  git bisect bad 6148219e90732fd06f5d7a498bda974e6a43ab4b
>>  # bad: [e0326ebe10191447ab8fa2e904080df7b743765e] Merge branch 'for-next' of https://git.kernel.org/pub/scm/linux/kernel/git/soc/soc.git
>>  git bisect bad e0326ebe10191447ab8fa2e904080df7b743765e
>>  # bad: [fbc9c5ac47cef5a2b04aef30c8e990b32dcf2548] Merge branch 'hwmon' of https://git.kernel.org/pub/scm/linux/kernel/git/groeck/linux-staging.git
>>  git bisect bad fbc9c5ac47cef5a2b04aef30c8e990b32dcf2548
>>  # bad: [e488171f6f6df6fc899a355079665fdb3c50b0e3] Merge branch 'for-linus' of https://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound.git
>>  git bisect bad e488171f6f6df6fc899a355079665fdb3c50b0e3
>>  # bad: [60db0fcb8fc9d80ac0b63041c632b41a311a45f1] Merge branch 'fs-current' of linux-next
>>  git bisect bad 60db0fcb8fc9d80ac0b63041c632b41a311a45f1
>>  # good: [51021d260d682aa17b3533848a99160ab83e0c93] Merge branch 'vfs.fixes' of https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
>>  git bisect good 51021d260d682aa17b3533848a99160ab83e0c93
>>  # good: [ded56474db6552260786a65898322464b72c7540] mm: a second pagecache maintainer
>>  git bisect good ded56474db6552260786a65898322464b72c7540
>>  # good: [6c893b948351d42cfc3761cc746ab5b3d03ee7f3] Merge branch 'misc-7.2' into next-fixes
>>  git bisect good 6c893b948351d42cfc3761cc746ab5b3d03ee7f3
>>  # good: [bfcc55a14179495b0c41408908fd7b9d7785c694] lib: test_hmm: use device devt for coherent device range selection
>>  git bisect good bfcc55a14179495b0c41408908fd7b9d7785c694
>>  # good: [a27318567c92ba5482906d047e71a7aa4fd01889] Merge branch 'fixes' of https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git
>>  git bisect good a27318567c92ba5482906d047e71a7aa4fd01889
>>  # bad: [6887a39652cdfd4cfd3b0962662c9cbc26ce5252] mm/page_vma_mapped: fix device-private PMD handling
>>  git bisect bad 6887a39652cdfd4cfd3b0962662c9cbc26ce5252
>>  # good: [2cc6bd0efc264b9ac760c2bc74dff4f521a680a1] MAINTAINERS: s/SeongJae/SJ/
>>  git bisect good 2cc6bd0efc264b9ac760c2bc74dff4f521a680a1
>>  # first 'bad' commit: [6887a39652cdfd4cfd3b0962662c9cbc26ce5252] mm/page_vma_mapped: fix device-private PMD handling
>> 
>>>
>>> Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries")
>>> Cc: <stable@vger.kernel.org>
>>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>>> Suggested-by: David Hildenbrand <david@kernel.org>
>>> Cc: David Hildenbrand <david@kernel.org>
>>> Cc: Balbir Singh <balbirs@nvidia.com>
>>> Cc: SeongJae Park <sj@kernel.org>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Cc: Lorenzo Stoakes <ljs@kernel.org>
>>> Cc: Lance Yang <lance.yang@linux.dev>
>>>
>>> ---
>>> v5:
>>>   * put device-private pmd handling along with the other two cases
>>>   * remove thp_migration_supported()
>>> v4: https://lore.kernel.org/all/20260624065353.1622-1-richard.weiyang@gmail.com/T/#u
>>>   * refine subject and commit log based on Lorenzo's suggestion
>>>   * put pmd device-private entry handling in its own if branch,
>>>     suggested by Lorenzo
>>>
>>> v3:
>>>   * remove cleanup part, only fix the issue for device-private entry
>>>   * refine user effect description based on Lorenzo's suggestion
>>>
>>> v2: https://lore.kernel.org/all/20260616063436.20455-1-richard.weiyang@gmail.com/T/#u
>>>   * specify the possible error case of current code and user visible effect
>>>   * besides fix, cleanup the pmd entry handling based on David's suggestion
>>>
>>> v1: https://lore.kernel.org/linux-mm/20260508013728.21285-1-richard.weiyang@gmail.com/
>>> ---
>>>  mm/page_vma_mapped.c | 30 ++++++++++++++++--------------
>>>  1 file changed, 16 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>> index 2ccbabfb2cc1..2d6c58488e3a 100644
>>> --- a/mm/page_vma_mapped.c
>>> +++ b/mm/page_vma_mapped.c
>>> @@ -243,21 +243,30 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>>  		 */
>>>  		pmde = pmdp_get_lockless(pvmw->pmd);
>>>  
>>> -		if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) {
>>> +		if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
>>> +		    pmd_is_device_private_entry(pmde)) {
>>>  			pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>>  			pmde = *pvmw->pmd;
>>> -			if (!pmd_present(pmde)) {
>>> +			if (pmd_is_migration_entry(pmde)) {
>>>  				softleaf_t entry;
>>>  
>>> -				if (!thp_migration_supported() ||
>>> -				    !(pvmw->flags & PVMW_MIGRATION))
>>> +				if (!(pvmw->flags & PVMW_MIGRATION))
>>>  					return not_found(pvmw);
>>>  				entry = softleaf_from_pmd(pmde);
>>> +				if (!check_pmd(softleaf_to_pfn(entry), pvmw))
>>> +					return not_found(pvmw);
>>> +				return true;
>>> +			} else if (pmd_is_device_private_entry(pmde)) {
>>> +				softleaf_t entry;
>>>  
>> 
>>> -				if (!softleaf_is_migration(entry) ||
>>> -				    !check_pmd(softleaf_to_pfn(entry), pvmw))
>> 
>> My only guess here would be that the compiler evaluates
>> !softleaf_is_migration(entry) to always be true and optimises away the
>> !check_pmd(softleaf_to_pfn(entry), pvmw) which is why this worked
>> before?
>
>Weird, we enter this path only with
>
>pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
>pmd_is_device_private_entry(pmde)
>
>If any one of these would compile for !CONFIG_TRANSPARENT_HUGEPAGE that would be
>odd.
>
>pmd_is_device_private_entry() is hard-coded to false unless
>CONFIG_ARCH_ENABLE_THP_MIGRATION. Which is only selected with
>ARCH_ENABLE_THP_MIGRATION.
>
>pmd_trans_huge() as well.
>
>Maybe it's struggling with pmd_is_migration_entry() on some (older) compilers?
>(not innlining stuff and not properly optimizing it out).
>
>The whole conditional must be optimized out.

Right. Kinda weird if compiler didn't fold

pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
pmd_is_device_private_entry(pmde)

away here ...

>We could check for IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) right at the start
>to make it easier for the compiler:

+1, explicit THP guard should do the trick :)

>if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) &&
>    (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
>     pmd_is_device_private_entry(pmde))) {
>
>

Klara, could you try with this change and see if it fixes the build?

Thanks, Lance