mm/mremap: correctly account old mapping after MREMAP_DONTUNMAP remap

Commit b714ccb ("mm/mremap: complete refactor of move_vma()")
mistakenly introduced a new behaviour - clearing the VM_ACCOUNT flag of
the old mapping when a mapping is mremap()'d with the MREMAP_DONTUNMAP
flag set.

While we always clear the VM_LOCKED and VM_LOCKONFAULT flags for the old
mapping (the page tables have been moved, so there is no data that could
possibly be locked in memory), there is no reason to touch any other VMA
flags.

This is because after the move the old mapping is in a state as if it were
freshly mapped.  This implies that the attributes of the mapping ought to
remain the same, including whether or not the mapping is accounted.

Link: https://lkml.kernel.org/r/20251013165836.273113-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Fixes: b714ccb ("mm/mremap: complete refactor of move_vma()")
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ljskernel

mm/mremap.c

@@ -1237,10 +1237,10 @@ static int copy_vma_and_data(struct vma_remap_struct *vrm,
 }
 
 /*
- * Perform final tasks for MADV_DONTUNMAP operation, clearing mlock() and
- * account flags on remaining VMA by convention (it cannot be mlock()'d any
- * longer, as pages in range are no longer mapped), and removing anon_vma_chain
- * links from it (if the entire VMA was copied over).
+ * Perform final tasks for MADV_DONTUNMAP operation, clearing mlock() flag on
+ * remaining VMA by convention (it cannot be mlock()'d any longer, as pages in
+ * range are no longer mapped), and removing anon_vma_chain links from it if the
+ * entire VMA was copied over.
  */
 static void dontunmap_complete(struct vma_remap_struct *vrm,
 			       struct vm_area_struct *new_vma)
@@ -1250,11 +1250,8 @@ static void dontunmap_complete(struct vma_remap_struct *vrm,
 	unsigned long old_start = vrm->vma->vm_start;
 	unsigned long old_end = vrm->vma->vm_end;
 
-	/*
-	 * We always clear VM_LOCKED[ONFAULT] | VM_ACCOUNT on the old
-	 * vma.
-	 */
-	vm_flags_clear(vrm->vma, VM_LOCKED_MASK | VM_ACCOUNT);
+	/* We always clear VM_LOCKED[ONFAULT] on the old VMA. */
+	vm_flags_clear(vrm->vma, VM_LOCKED_MASK);
 
 	/*
 	 * anon_vma links of the old vma is no longer needed after its page