mm/vmalloc: Warn on improper use of vunmap_range()

A call to vmalloc_huge() may cause memory blocks to be mapped at pmd or
pud level. But it is possible to subsequently call vunmap_range() on a
sub-range of the mapped memory, which partially overlaps a pmd or pud.
In this case, vmalloc unmaps the entire pmd or pud so that the
no-overlapping portion is also unmapped. Clearly that would have a bad
outcome, but it's not something that any callers do today as far as I
can tell. So I guess it's just expected that callers will not do this.

However, it would be useful to know if this happened in future; let's
add a warning to cover the eventuality.

Reviewed-by: Anshuman Khandual <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Reviewed-by: Uladzislau Rezki (Sony) <[email protected]>
Signed-off-by: Ryan Roberts <[email protected]>
Tested-by: Luiz Capitulino <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: Ryan Roberts

mm/vmalloc.c

@@ -374,8 +374,10 @@ static void vunmap_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
 		if (cleared || pmd_bad(*pmd))
 			*mask |= PGTBL_PMD_MODIFIED;
 
-		if (cleared)
+		if (cleared) {
+			WARN_ON(next - addr < PMD_SIZE);
 			continue;
+		}
 		if (pmd_none_or_clear_bad(pmd))
 			continue;
 		vunmap_pte_range(pmd, addr, next, mask);
@@ -399,8 +401,10 @@ static void vunmap_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
 		if (cleared || pud_bad(*pud))
 			*mask |= PGTBL_PUD_MODIFIED;
 
-		if (cleared)
+		if (cleared) {
+			WARN_ON(next - addr < PUD_SIZE);
 			continue;
+		}
 		if (pud_none_or_clear_bad(pud))
 			continue;
 		vunmap_pmd_range(pud, addr, next, mask);