* [PATCH] mm/migrate_device: document folio_get requirement before frozen PMD split
@ 2026-03-06 10:44 Usama Arif
2026-03-06 12:18 ` Zi Yan
0 siblings, 1 reply; 2+ messages in thread
From: Usama Arif @ 2026-03-06 10:44 UTC (permalink / raw)
To: Andrew Morton, npache, david, ziy, linux-mm, mpenttil, balbirs
Cc: matthew.brost, joshua.hahnjy, hannes, rakie.kim, byungchul,
gourry, ying.huang, apopple, riel, shakeel.butt, kas,
linux-kernel, kernel-team, Usama Arif
split_huge_pmd_address() with freeze=true splits a PMD migration entry
into PTE migration entries, consuming one folio reference in the
process. The folio_get() before it provides this reference.
Add a comment explaining this relationship and a VM_WARN_ON_ONCE to
catch an unexpected refcount != 1 entry state.
Suggested-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
mm/migrate_device.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 78c7acf024615..6fa2878848a7e 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -908,6 +908,11 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
unsigned long flags;
int ret = 0;
+ VM_WARN_ON_ONCE(folio_ref_count(folio) != 1);
+ /*
+ * take a reference, since split_huge_pmd_address() with freeze = true
+ * drops a reference at the end.
+ */
folio_get(folio);
split_huge_pmd_address(migrate->vma, addr, true);
ret = folio_split_unmapped(folio, 0);
--
2.47.3
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] mm/migrate_device: document folio_get requirement before frozen PMD split
2026-03-06 10:44 [PATCH] mm/migrate_device: document folio_get requirement before frozen PMD split Usama Arif
@ 2026-03-06 12:18 ` Zi Yan
0 siblings, 0 replies; 2+ messages in thread
From: Zi Yan @ 2026-03-06 12:18 UTC (permalink / raw)
To: Usama Arif
Cc: Andrew Morton, npache, david, linux-mm, mpenttil, balbirs,
matthew.brost, joshua.hahnjy, hannes, rakie.kim, byungchul,
gourry, ying.huang, apopple, riel, shakeel.butt, kas,
linux-kernel, kernel-team
On 6 Mar 2026, at 5:44, Usama Arif wrote:
> split_huge_pmd_address() with freeze=true splits a PMD migration entry
> into PTE migration entries, consuming one folio reference in the
> process. The folio_get() before it provides this reference.
>
> Add a comment explaining this relationship and a VM_WARN_ON_ONCE to
> catch an unexpected refcount != 1 entry state.
>
> Suggested-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
> mm/migrate_device.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
Thanks for fixing the logic in my suggestion,
Reviewed-by: Zi Yan <ziy@nvidia.com>
Best Regards,
Yan, Zi
^ permalink raw reply [flat|nested] 2+ messages in thread
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