fuse: dax: do not set VM_MIXEDMAP

Commit e1fb4a0864 ("dax: remove VM_MIXEDMAP for fsdax and device dax")
prevented fsdax and device-dax from setting VM_MIXEDMAP, as DAX no longer
relies on it to direct core mm paths.

The fuse DAX implementation, added later, copied the old pattern and still
sets it.

Fuse DAX maps pages the same way fsdax does, via dax_iomap_fault() and
ultimately vmf_insert_page_mkwrite() and vmf_insert_folio_pmd(), which
insert ordinary refcounted pages and so do not require VM_MIXEDMAP.

Setting it only serves to mark the mapping as not mm-managed, making fuse
DAX the sole DAX implementation whose mappings are unmergeable, cannot be
mlock()'d, eagerly copy page tables on fork and reject MADV_DOFORK and
MADV_DODUMP.

It also requires vma_is_special_huge() in mm/huge_memory.c to carve DAX out
of its check for mappings not managed by core mm explicitly.

There is no reason for fuse DAX to keep on using this flag so drop it.

Link: https://lore.kernel.org/20261013-b4-mmap-prepare-vma-flag-sanify-v6-38-1aa3c9b139f9@kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
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fs/fuse/dax.c

@@ -826,7 +826,7 @@ int fuse_dax_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	file_accessed(file);
 	vma->vm_ops = &fuse_dax_vm_ops;
-	vm_flags_set(vma, VM_MIXEDMAP | VM_HUGEPAGE);
+	vma_set_flags(vma, VMA_HUGEPAGE_BIT);
 	return 0;
 }