mm: further refactor commit_merge()

The current VMA merge mechanism contains a number of confusing mechanisms
around removal of VMAs on merge and the shrinking of the VMA adjacent to
vma->target in the case of merges which result in a partial merge with
that adjacent VMA.

Since we now have a STABLE set of VMAs - prev, middle, next - we are now
able to have the caller of commit_merge() explicitly tell us which VMAs
need deleting, using newly introduced internal VMA merge flags.

Doing so allows us to embed this state within the VMG and remove the
confusing remove, remove2 parameters from commit_merge().

We additionally are able to eliminate the highly confusing and misleading
'expanded' parameter - a parameter that in reality refers to whether or
not the return VMA is the target one or the one immediately adjacent.

We can infer which is the case from whether or not the adj_start parameter
is negative.  This also allows us to simplify further logic around
iterator configuration and VMA iterator stores.

Doing so means we can also eliminate the adjust parameter, as we are able
to infer which VMA ought to be adjusted from adj_start - a positive value
implies we adjust the start of 'middle', a negative one implies we adjust
the start of 'next'.

We are then able to have commit_merge() explicitly return the target VMA,
or NULL on inability to pre-allocate memory.  Errors were previously
filtered so behaviour does not change.

We additionally move from the slightly odd use of a bitwise-flag enum
vmg->merge_flags field to vmg bitfields.

This patch has no change in functional behaviour.

Link: https://lkml.kernel.org/r/7bf2ed24af68aac18672b7acebbd9102f48c5b03.1738326519.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 24 insertions, 14 deletions

diff --git a/mm/vma.h b/mm/vma.h
index 5b5dd07e478c..7935681a2db8 100644
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -58,17 +58,6 @@ enum vma_merge_state {
 	VMA_MERGE_SUCCESS,
 };
 
-enum vma_merge_flags {
-	VMG_FLAG_DEFAULT = 0,
-	/*
-	 * If we can expand, simply do so. We know there is nothing to merge to
-	 * the right. Does not reset state upon failure to merge. The VMA
-	 * iterator is assumed to be positioned at the previous VMA, rather than
-	 * at the gap.
-	 */
-	VMG_FLAG_JUST_EXPAND = 1 << 0,
-};
-
 /*
  * Describes a VMA merge operation and is threaded throughout it.
  *
@@ -117,8 +106,31 @@ struct vma_merge_struct {
 	struct mempolicy *policy;
 	struct vm_userfaultfd_ctx uffd_ctx;
 	struct anon_vma_name *anon_name;
-	enum vma_merge_flags merge_flags;
 	enum vma_merge_state state;
+
+	/* Flags which callers can use to modify merge behaviour: */
+
+	/*
+	 * If we can expand, simply do so. We know there is nothing to merge to
+	 * the right. Does not reset state upon failure to merge. The VMA
+	 * iterator is assumed to be positioned at the previous VMA, rather than
+	 * at the gap.
+	 */
+	bool just_expand :1;
+
+	/* Internal flags set during merge process: */
+
+	/*
+	 * Internal flag used during the merge operation to indicate we will
+	 * remove vmg->middle.
+	 */
+	bool __remove_middle :1;
+	/*
+	 * Internal flag used during the merge operationr to indicate we will
+	 * remove vmg->next.
+	 */
+	bool __remove_next :1;
+
 };
 
 static inline bool vmg_nomem(struct vma_merge_struct *vmg)
@@ -142,7 +154,6 @@ static inline pgoff_t vma_pgoff_offset(struct vm_area_struct *vma,
 		.flags = flags_,					\
 		.pgoff = pgoff_,					\
 		.state = VMA_MERGE_START,				\
-		.merge_flags = VMG_FLAG_DEFAULT,			\
 	}
 
 #define VMG_VMA_STATE(name, vmi_, prev_, vma_, start_, end_)	\
@@ -162,7 +173,6 @@ static inline pgoff_t vma_pgoff_offset(struct vm_area_struct *vma,
 		.uffd_ctx = vma_->vm_userfaultfd_ctx,		\
 		.anon_name = anon_vma_name(vma_),		\
 		.state = VMA_MERGE_START,			\
-		.merge_flags = VMG_FLAG_DEFAULT,		\
 	}
 
 #ifdef CONFIG_DEBUG_VM_MAPLE_TREE