[PATCH V3 2/5] mm/khugepaged: count small VMAs towards scan limit

[Shivank Garg <shivankg@amd.com>]

The khugepaged_scan_mm_slot() uses a 'progress' counter to limit the
amount of work performed and consists of three components:
1. Transitioning to a new mm (+1).
2. Skipping an unsuitable VMA (+1).
3. Scanning a PMD-sized range (+HPAGE_PMD_NR).

Consider a 1MB VMA sitting between two 2MB alignment boundaries:

     vma1       vma2   vma3
    +----------+------+----------+
    |2M        |1M    |2M        |
    +----------+------+----------+
               ^      ^
               start  end
               ^
          hstart,hend

In this case, for vma2:
  hstart = round_up(start, HPAGE_PMD_SIZE)  -> Next 2MB alignment
  hend   = round_down(end, HPAGE_PMD_SIZE) -> Prev 2MB alignment

Currently, since `hend <= hstart`, VMAs that are too small or unaligned
to contain a hugepage are skipped without incrementing 'progress'.
A process containing a large number of such small VMAs will unfairly
consume more CPU cycles before yielding compared to a process with
fewer, larger, or aligned VMAs.

Fix this by incrementing progress when the `hend <= hstart` condition
is met.

Additionally, change 'progress' type to `unsigned int` to match both
the 'pages' type and the function return value.

Suggested-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Shivank Garg <shivankg@amd.com>
---

Incorporate comment feedback from Lance:
https://lore.kernel.org/linux-mm/6b408736-978a-4d40-adfc-97819951c3a6@linux.dev

 mm/khugepaged.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 984294a16861..93ce39915f4a 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2403,7 +2403,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 	struct mm_slot *slot;
 	struct mm_struct *mm;
 	struct vm_area_struct *vma;
-	int progress = 0;
+	unsigned int progress = 0;
 
 	VM_BUG_ON(!pages);
 	lockdep_assert_held(&khugepaged_mm_lock);
@@ -2447,7 +2447,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 		}
 		hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
 		hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
-		if (khugepaged_scan.address > hend) {
+		if (khugepaged_scan.address > hend || hend <= hstart) {
+			/* VMA already scanned or too small/unaligned for hugepage. */
 			progress++;
 			continue;
 		}
-- 
2.43.0