Re: [RFC PATCH 2/3] mm/huge_memory: add kernel-doc for folio_split_supported()
From: David Hildenbrand (Red Hat)
Date: Thu Nov 20 2025 - 15:01:58 EST
On 11/20/25 15:48, Zi Yan wrote:
On 20 Nov 2025, at 4:27, David Hildenbrand (Red Hat) wrote:
On 11/20/25 04:59, Zi Yan wrote:
It clarifies that folio_split_supported() does not check folio->mapping and
can dereference it.
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
mm/huge_memory.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index efea42d68157..15e555f1b85d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3688,6 +3688,23 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
return 0;
}
+/**
+ * folio_split_supported() - check if a folio can be split to a given order
+ * @folio: folio to be split
+ * @new_order: the smallest order of the after split folios (since buddy
+ * allocator like split generates folios with orders from @folio's
+ * order - 1 to new_order).
+ * @split_type: uniform or non-uniform split
+ * @warns: whether gives warnings or not for the checks in the function
+ *
+ * folio_split_supported() checks if @folio can be split to @new_order using
+ * @split_type method.
+ *
+ * Context: Caller must make sure folio->mapping is not NULL, since the
+ * function does not check it and can dereference folio->mapping
Only for anon folios. Also, I would drop the detail about dereference.
OK.
I guess we really need the folio lock to prevent concurrent truncation.
Maybe something like:
"The folio must be locked. For non-anon folios, the caller must make sure that folio->mapping is not NULL (e.g., not truncated)."
Sure. Do you think it is worth adding VM_WARN_ONCE_ON(!folio_test_locked);
and VM_WARN_ONCE_ON(!folio->mapping); ?
Makes sense. Or we allow !folio->mapping, return false and do something like the following. Still wondering how we could handle that case better.
if (!folio_split_supported(folio)) {
if (folio_split_temporarily_unsupported(folio))
return -EBUSY;
return -EINVAL;
}
hmmmm
--
Cheers
David