[PATCH 22/39] mm/mlock: clear VMA_LOCKED_MASK over mmap callback

[Lorenzo Stoakes (ARM)]

Currently there's a confusing mess around VMA_LOCKED_BIT and
VMA_LOCKONFAULT_BIT.

It is permitted for drivers to set any flags they like, with the VMA
already possessing lock flags.

This results in the absurd situation of a VMA possessing both
VMA_SPECIAL_FLAGS and VMA_LOCKED_MASK flags, which is not permitted.

This has resulted in mlock_vma_folio() having a very silly check for this
scenario to work around it.

There is no need for this - just clear the flags before invoking the hook
and reinstate them afterwards if they are required.

Nothing relies upon this being set during the mmap operation.

mmap_prepare is unaffected by this so requires no fix.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
---
 mm/internal.h |  9 +--------
 mm/vma.c      | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index a86b9803a4c3..6e27d3b10c01 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -975,14 +975,7 @@ void mlock_folio(struct folio *folio);
 static inline void mlock_vma_folio(struct folio *folio,
 				struct vm_area_struct *vma)
 {
-	/*
-	 * The VM_SPECIAL check here serves two purposes.
-	 * 1) VM_IO check prevents migration from double-counting during mlock.
-	 * 2) Although mmap_region() and mlock_fixup() take care that VM_LOCKED
-	 *    is never left set on a VM_SPECIAL vma, there is an interval while
-	 *    file->f_op->mmap() is using vm_insert_page(s), when VM_LOCKED may
-	 *    still be set while VM_SPECIAL bits are added: so ignore it then.
-	 */
+	/* The VM_IO check prevents migration from double-counting during mlock. */
 	if (unlikely((vma->vm_flags & (VM_LOCKED|VM_SPECIAL)) == VM_LOCKED))
 		mlock_folio(folio);
 }
diff --git a/mm/vma.c b/mm/vma.c
index 526428753218..cb0c4c625756 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -2594,6 +2594,11 @@ static int __mmap_new_file_vma(struct mmap_state *map,
 	if (!map->file->f_op->mmap)
 		return 0;
 
+	/*
+	 * Driver-specified flags may make the lock flags invalid, so clear
+	 * VMA_LOCKED_MASK and reinstate it afterwards if appropriate.
+	 */
+	vma_clear_flags_mask(vma, VMA_LOCKED_MASK);
 	error = mmap_file(vma->vm_file, vma);
 	if (error) {
 		UNMAP_STATE(unmap, vmi, vma, vma->vm_start, vma->vm_end,
@@ -2607,6 +2612,15 @@ static int __mmap_new_file_vma(struct mmap_state *map,
 		return error;
 	}
 
+	/* If VMA flags still valid for locked mask, reinstate. */
+	if (vma_supports_mlock(vma)) {
+		const vma_flags_t mask =
+			vma_flags_and_mask(&map->vma_flags,
+					   VMA_LOCKED_MASK);
+
+		vma_set_flags_mask(vma, mask);
+	}
+
 	map->file = vma->vm_file;
 	map->vma_flags = vma->flags;
 

-- 
2.55.0