[PATCH v2] mm/khugepaged: avoid underflow in madvise_collapse for sub-PMD MADV_COLLAPSE

[Wandun Chen <chenwandun1@gmail.com>]

From: Chen Wandun <chenwandun@lixiang.com>

madvise_collapse() computes the THP-aligned window:

    hstart = ALIGN(start, HPAGE_PMD_SIZE); 	/* round up */
    hend = ALIGN_DOWN(end, HPAGE_PMD_SIZE);	/* round down */

The following case will cause hstart > hend, and result in underflow
in the return statement, avoid it by returning -EINVAL early when
hstart > hend.

    madvise(PMD-aligned + PAGE_SIZE, PAGE_SIZE, MADV_COLLAPSE);

In addition, kmalloc_obj(), mmgrab() and lru_add_drain_all() are
unnecessary when hstart == hend, so skip these operations by
returning early too.

Signed-off-by: Chen Wandun <chenwandun@lixiang.com>

---
v1 --> v2:
- Rebase and resolve code conflict.
- Return -EINVAL when hstart > hend, suggested by Lorenzo.
- Drop Fixes tag, suggested by David and Lorenzo.
- Updated commit message to be more explicit, suggested by Lorenzo.
---
 mm/khugepaged.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 28a843f30b32..36baab17f098 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2837,6 +2837,15 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 	if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_FORCED_COLLAPSE, PMD_ORDER))
 		return -EINVAL;
 
+	hstart = ALIGN(start, HPAGE_PMD_SIZE);
+	hend = ALIGN_DOWN(end, HPAGE_PMD_SIZE);
+
+	if (hstart > hend)
+		return -EINVAL;
+
+	if (hstart == hend)
+		return 0;
+
 	cc = kmalloc_obj(*cc);
 	if (!cc)
 		return -ENOMEM;
@@ -2846,9 +2855,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 	mmgrab(mm);
 	lru_add_drain_all();
 
-	hstart = ALIGN(start, HPAGE_PMD_SIZE);
-	hend = ALIGN_DOWN(end, HPAGE_PMD_SIZE);
-
 	for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
 		enum scan_result result = SCAN_FAIL;
 
-- 
2.43.0