[RFC PATCH 3/3] mm/vma: remove mmap() retry merge

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

We have now introduced a mechanism that obviates the need for a reattempted
merge via the f_op->mmap_proto() hook, so eliminate this functionality
altogether.

The retry merge logic has been the cause of a great deal of complexity in
the past and required a great deal of careful manoeuvring of code to ensure
its continued and correct functionality.

It has also recently been involved in an issue surrounding maple tree
state, which again points to its problematic nature.

We make it much easier to reason about mmap() logic by eliminating this and
simply writing a VMA once. This also opens the doors to future
optimisation and improvement in the mmap() logic.

For any device or file system which encounters unwanted VMA fragmentation
as a result of this change (that is, having not implemented .mmap_proto
hooks), the issue is easily resolvable by doing so.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/vma.c | 14 --------------
 1 file changed, 14 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index 76bd3a67ce0f..40c98f88472e 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -24,7 +24,6 @@ struct mmap_state {
 	void *vm_private_data;
 
 	unsigned long charged;
-	bool retry_merge;
 
 	struct vm_area_struct *prev;
 	struct vm_area_struct *next;
@@ -2423,8 +2422,6 @@ static int __mmap_new_file_vma(struct mmap_state *map,
 			!(map->flags & VM_MAYWRITE) &&
 			(vma->vm_flags & VM_MAYWRITE));
 
-	/* If the flags change (and are mergeable), let's retry later. */
-	map->retry_merge = vma->vm_flags != map->flags && !(vma->vm_flags & VM_SPECIAL);
 	map->flags = vma->vm_flags;
 
 	return 0;
@@ -2641,17 +2638,6 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
 	if (have_proto)
 		set_vma_user_defined_fields(vma, &map);
 
-	/* If flags changed, we might be able to merge, so try again. */
-	if (map.retry_merge) {
-		struct vm_area_struct *merged;
-		VMG_MMAP_STATE(vmg, &map, vma);
-
-		vma_iter_config(map.vmi, map.addr, map.end);
-		merged = vma_merge_existing_range(&vmg);
-		if (merged)
-			vma = merged;
-	}
-
 	__mmap_complete(&map, vma);
 
 	return addr;
-- 
2.49.0