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mm/migrate_device: document folio_get requirement before frozen PMD split

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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.  The expected folio refcount
at the start of migrate_vma_split_unmapped_folio() is 1.

Link: https://lkml.kernel.org/r/20260309212502.3922825-1-usama.arif@linux.dev
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Suggested-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Nico Pache <npache@redhat.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Usama Arif
2026-03-09 14:25:02 -07:00
committed by Andrew Morton
parent d765108993
commit 5a14198ec6
1 changed files with 4 additions and 0 deletions
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4
mm/migrate_device.c
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@@ -914,6 +914,10 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
unsigned long flags;
int ret = 0;
/*
* 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);