[PATCH] mm/MADV_COLLAPSE: don't expand collapse when vm_end is past requested end

["Zach O'Keefe" <zokeefe@google.com>]

MADV_COLLAPSE acts on one hugepage-aligned/sized region at a time, until
it has collapsed all eligible memory contained within the bounds
supplied by the user.

At the top of each hugepage iteration we (re)lock mmap_lock and
(re)validate the VMA for eligibility and update variables that might
have changed while mmap_lock was dropped.  One thing that might occur,
is that the VMA could be resized, and as such, we refetch vma->vm_end
to make sure we don't collapse past the end of the VMA.

However, it's possible that during this refetch that we expand the
region acted on by MADV_COLLAPSE if vma->vm_end is greater than the end
of the user-supplied range.

Don't expand the acted-on region when refetching vma->vm_end.

Fixes: 4d24de9425f7 ("mm: MADV_COLLAPSE: refetch vm_end after reacquiring mmap_lock")
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
Cc: Yang Shi <shy828301@gmail.com>
---
 mm/khugepaged.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5cb401aa2b9d..b4d2ec0a94ed 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2649,7 +2649,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
 				goto out_nolock;
 			}
 
-			hend = vma->vm_end & HPAGE_PMD_MASK;
+			hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
 		}
 		mmap_assert_locked(mm);
 		memset(cc->node_load, 0, sizeof(cc->node_load));
-- 
2.39.0.314.g84b9a713c41-goog