[PATCH v2 1/4] mm: drop the huge PMD assertion from walk_to_pmd()

[Eliav Farber]

walk_to_pmd() asserts that the PMD it returns is not a huge entry. That
held when commit c2febafc6773 ("mm: convert generic code to 5-level
paging") added the assertion: nothing installed a huge PMD over the
mappings its callers walk.

ARCH_SUPPORTS_HUGE_PFNMAP changed that. A VM_PFNMAP mapping whose driver
provides both ->fault and ->huge_fault can have a PMD leaf installed by a
concurrent huge fault over the same address, so a caller can observe one
and BUG on CONFIG_DEBUG_VM=y.

The walk does not need the invariant. pmd_alloc() only allocates when the
PUD is empty and otherwise returns pmd_offset(), so a leaf is returned
rather than overwritten. Every caller already copes with one, because
they all reach pte_offset_map_lock(), which refuses to walk a leaf and
returns NULL: insert_pages() turns that into -EFAULT, get_locked_pte()
passes the NULL to its callers, and insert_pfn() returns a fault code.

Fixes: f9e54c3a2f5b ("vfio/pci: implement huge_fault support")
Signed-off-by: Eliav Farber <farbere@xxxxxxxxxx>
Reviewed-by: Jose A. Perez de Azpillaga <azpijr@xxxxxxxxx>
---
 mm/memory.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index ec3d9d99ccee..0f54ee0a6d88 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2384,7 +2384,6 @@ static pmd_t *walk_to_pmd(struct mm_struct *mm, unsigned long addr)
 	if (!pmd)
 		return NULL;
 
-	VM_BUG_ON(pmd_trans_huge(*pmd));
 	return pmd;
 }
 
-- 
2.47.3