[PATCH 6.17 1/3] mm/mremap: allow multi-VMA move when filesystem uses thp_get_unmapped_area

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

We currently restrict multi-VMA move to avoid filesystems or drivers which
provide a custom f_op->get_unmapped_area handler unless it is known to
correctly handle MREMAP_FIXED.

We do this so we do not get unexpected result when moving from one area to
another (for instance, if the handler would align things resulting in the
moved VMAs having different gaps than the original mapping).

More and more filesystems are moving to using large folios, and typically
do so (in part) by setting f_op->get_unmapped_area to
thp_get_unmapped_area.

When mremap() invokes the file system's get_unmapped MREMAP_FIXED, it does
so via get_unmapped_area(), called in vrm_set_new_addr(). In order to do
so, it converts the MREMAP_FIXED flag to a MAP_FIXED flag and passes this
to the unmapped area handler.

The __get_unmapped_area() function (called by get_unmapped_area()) in turn
invokes the filesystem or driver's f_op->get_unmapped_area() handler.

Therefore this is a point at which thp_get_unmapped_area() may be called
(also, this is the case for anonymous mappings where the size is huge page
aligned).

thp_get_unmapped_area() calls thp_get_unmapped_area_vmflags() and
__thp_get_unmapped_area() in turn (falling back to
mm_get_unmapped_area_vm_flags() which is known to handle MAP_FIXED
correctly).

The __thp_get_unmapped_area() function in turn does nothing to change the
address hint, nor the MAP_FIXED flag, only adjusting alignment
parameters. It hten calls mm_get_unmapped_area_vmflags(), and in turn
arch-specific unmapped area functions, all of which honour MAP_FIXED
correctly.

Therefore, we can safely add thp_get_unmapped_area to the known-good
handlers.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/mremap.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 677a4d744df9..46f9f3160dff 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1616,7 +1616,7 @@ static void notify_uffd(struct vma_remap_struct *vrm, bool failed)
 
 static bool vma_multi_allowed(struct vm_area_struct *vma)
 {
-	struct file *file;
+	struct file *file = vma->vm_file;
 
 	/*
 	 * We can't support moving multiple uffd VMAs as notify requires
@@ -1629,15 +1629,17 @@ static bool vma_multi_allowed(struct vm_area_struct *vma)
 	 * Custom get unmapped area might result in MREMAP_FIXED not
 	 * being obeyed.
 	 */
-	file = vma->vm_file;
-	if (file && !vma_is_shmem(vma) && !is_vm_hugetlb_page(vma)) {
-		const struct file_operations *fop = file->f_op;
-
-		if (fop->get_unmapped_area)
-			return false;
-	}
+	if (!file || !file->f_op->get_unmapped_area)
+		return true;
+	/* Known good. */
+	if (vma_is_shmem(vma))
+		return true;
+	if (is_vm_hugetlb_page(vma))
+		return true;
+	if (file->f_op->get_unmapped_area == thp_get_unmapped_area)
+		return true;
 
-	return true;
+	return false;
 }
 
 static int check_prep_vma(struct vma_remap_struct *vrm)
-- 
2.50.1