[PATCH 1/2] mm/vmalloc: drop VM_DMA_COHERENT from remap_vmalloc_range_partial

[Ye Liu <ye.liu@linux.dev>]

From: Ye Liu <liuye@kylinos.cn>

No caller maps a VM_DMA_COHERENT area through this path: such areas
come from dma_alloc_coherent() internals and are freed via vunmap(),
never through remap_vmalloc_range[_partial](); all 13 callers pass
vmalloc_user()/vmalloc()-backed addresses.

The flag was added in commit fe9041c245196 ("vmalloc: lift the arm
flag for coherent mappings to common code") as a mechanical
translation of ARM's VM_ARM_DMA_CONSISTENT. It is also unsafe to keep
since commit 5ea8ec74c57c0 ("mm/vmalloc: free unused pages on
vrealloc() shrink"), which frees tail pages of non-VM_USERMAP areas
(VM_DMA_COHERENT is not in that skip-list): a shrunk-then-mapped area
would hit vmalloc_to_page()==NULL and vm_insert_page() would
dereference it unchecked.

Drop it so DMA-coherent areas can never enter the path.

Signed-off-by: Ye Liu <liuye@kylinos.cn>
---
 mm/vmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index db669103dc66..a888eca016a8 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4894,7 +4894,7 @@ int remap_vmalloc_range_partial(struct vm_area_struct *vma, unsigned long uaddr,
 	if (!area)
 		return -EINVAL;
 
-	if (!(area->flags & (VM_USERMAP | VM_DMA_COHERENT)))
+	if (!(area->flags & VM_USERMAP))
 		return -EINVAL;
 
 	if (check_add_overflow(size, off, &end_index) ||

-- 
2.43.0