Re: [PATCH v2 0/2] mm: fix PMD level mTHP accounting bugs

[David Hildenbrand (Arm)]

On 7/21/26 15:57, Nico Pache wrote:
> On Fri, Jul 17, 2026 at 7:29 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> On Fri, 17 Jul 2026 00:44:58 -0600 Nico Pache <npache@xxxxxxxxxx> wrote:
>>
>>> While running selftests I noticed the PMD level per-mTHP stats (nr_anon)
>>> remained elevated after each run. After further investigation I noticed
>>> this accounting error occurs for both the migration.private_anon_htlb_test
>>> and the HMM tests.
>>>
>>> In the HMM case this is due to folio_add_new_anon_rmap() incrementing the
>>> mTHP stats, but never containing a corresponding decrement in
>>> free_zone_device_folio(). We solve this by making sure to decrement the
>>> counter when freeing device memory.
>>>
>>> In the migration case, we are incrementing this counter without first
>>> checking whether this folio is a hugetlb folio, which relies on a separate
>>> accounting system. We solve this by adding the proper hugetlb check before
>>> incrementing this counter.
>>>
>>> With these changes in place, the two tests no longer cause elevated PMD
>>> level accounting issues.
>>
>> Thanks, I've updated mm.git's mm-hotfixes-unstable branch to this
>> version.
> 
> Thanks!
> 
>>
>> Sashiko is worried about the existing code ("list corruption or a
>> use-after-free panic"):
>>         https://sashiko.dev/#/patchset/20260717064502.1980173-1-npache@xxxxxxxxxx
> 
> https://sashiko.dev/#/patchset/20260717064502.1980173-1-npache@xxxxxxxxxx
> 
> I dug into this, although I'm not very familiar with device memory.

In general: Don't let a tool waste your time on unrelated things if it wasn't
you that asked the tool for it. Someone else asked the tool to find unrelated
things.

I thought we only support anonymous THP for device-private, not device-coherent.

Maybe lib/test_hmm.c could somehow trigger it by selecting
MIGRATE_VMA_SELECT_COMPOUND?

But in migrate_device_coherent_folio() we have

	WARN_ON_ONCE(folio_test_large(folio));

So this would already be pretty broken and I wouldn't spend any more time on it.

-- 
Cheers,

David