Re: [PATCH mm-unstable v18 04/14] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

generalize the order of the __collapse_huge_page_* and collapse_max_*
functions to support future mTHP collapse.

The current mechanism for determining collapse with the
khugepaged_max_ptes_none value is not designed with mTHP in mind. This
raises a key design issue: if we support user defined max_pte_none values
(even those scaled by order), a collapse of a lower order can introduces
an feedback loop, or "creep", when max_ptes_none is set to a value greater
than HPAGE_PMD_NR / 2. [1]

With this configuration, a successful collapse to order N will populate
enough pages to satisfy the collapse condition on order N+1 on the next
scan. This leads to unnecessary work and memory churn.

To fix this issue introduce a helper function that will limit mTHP
collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
This effectively supports two modes: [2]

- max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE
  that maps the shared zeropage. Consequently, no memory bloat.
- max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
  available mTHP order.

This removes the possibility of "creep", and a warning will be emitted if
any non-supported max_ptes_none value is configured with mTHP enabled.
Any intermediate value will default mTHP collapse to max_ptes_none=0.

mTHP collapse will not honor the khugepaged_max_ptes_shared or
khugepaged_max_ptes_swap parameters, and will fail if it encounters a
shared or swapped entry.

No functional changes in this patch; however it defines future behavior
for mTHP collapse.

[1] - https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com (local)
[2] - https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com (local)

Reviewed-by: Lance Yang <lance.yang@linux.dev>
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <redacted>
---
 mm/khugepaged.c | 121 +++++++++++++++++++++++++++++++++++-------------
 1 file changed, 88 insertions(+), 33 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 116f39518948..e98ba5b15163 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c

@@ -353,30 +353,52 @@ static bool pte_none_or_zero(pte_t pte)
  * the shared zeropage for the given collapse operation.
  * @cc: The collapse control struct
  * @vma: The vma to check for userfaultfd
+ * @order: The folio order being collapsed to
  *
  * Return: Maximum number of empty/shared zeropage PTEs for the collapse operation
  */
 static unsigned int collapse_max_ptes_none(struct collapse_control *cc,
-		struct vm_area_struct *vma)
+		struct vm_area_struct *vma, unsigned int order)
 {
+	unsigned int max_ptes_none = khugepaged_max_ptes_none;