mm/mremap: refactor move_page_tables(), abstracting state

A lot of state is threaded throughout the page table moving logic within
the mremap code, including boolean values which control behaviour
specifically in regard to whether rmap locks need be held over the
operation and whether the VMA belongs to a temporary stack being moved by
move_arg_pages() (and consequently, relocate_vma_down()).

As we already transmit state throughout this operation, it is neater and
more readable to maintain a small state object.  We do so in the form of
pagetable_move_control.

In addition, this allows us to update parameters within the state as we
manipulate things, for instance with regard to the page table realignment
logic.

In future I want to add additional functionality to the page table logic,
so this is an additional motivation for making it easier to do so.

This patch changes move_page_tables() to accept a pointer to a
pagetable_move_control struct, and performs changes at this level only.
Further page table logic will be updated in a subsequent patch.

We additionally also take the opportunity to add significant comments
describing the address realignment logic to make it abundantly clear what
is going on in this code.

Link: https://lkml.kernel.org/r/e20180add9c8746184aa3f23a61fff69a06cdaa9.1741639347.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 39 insertions, 4 deletions

diff --git a/mm/internal.h b/mm/internal.h
index aa30282a774a..06f816cdb43b 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -25,6 +25,44 @@
 struct folio_batch;
 
 /*
+ * Maintains state across a page table move. The operation assumes both source
+ * and destination VMAs already exist and are specified by the user.
+ *
+ * Partial moves are permitted, but the old and new ranges must both reside
+ * within a VMA.
+ *
+ * mmap lock must be held in write and VMA write locks must be held on any VMA
+ * that is visible.
+ *
+ * Use the PAGETABLE_MOVE() macro to initialise this struct.
+ *
+ * NOTE: The page table move is affected by reading from [old_addr, old_end),
+ * and old_addr may be updated for better page table alignment, so len_in
+ * represents the length of the range being copied as specified by the user.
+ */
+struct pagetable_move_control {
+	struct vm_area_struct *old; /* Source VMA. */
+	struct vm_area_struct *new; /* Destination VMA. */
+	unsigned long old_addr; /* Address from which the move begins. */
+	unsigned long old_end; /* Exclusive address at which old range ends. */
+	unsigned long new_addr; /* Address to move page tables to. */
+	unsigned long len_in; /* Bytes to remap specified by user. */
+
+	bool need_rmap_locks; /* Do rmap locks need to be taken? */
+	bool for_stack; /* Is this an early temp stack being moved? */
+};
+
+#define PAGETABLE_MOVE(name, old_, new_, old_addr_, new_addr_, len_)	\
+	struct pagetable_move_control name = {				\
+		.old = old_,						\
+		.new = new_,						\
+		.old_addr = old_addr_,					\
+		.old_end = (old_addr_) + (len_),			\
+		.new_addr = new_addr_,					\
+		.len_in = len_,						\
+	}
+
+/*
  * The set of flags that only affect watermark checking and reclaim
  * behaviour. This is used by the MM to obey the caller constraints
  * about IO, FS and watermark checking while ignoring placement
@@ -1527,10 +1565,7 @@ extern struct list_lru shadow_nodes;
 } while (0)
 
 /* mremap.c */
-unsigned long move_page_tables(struct vm_area_struct *vma,
-	unsigned long old_addr, struct vm_area_struct *new_vma,
-	unsigned long new_addr, unsigned long len,
-	bool need_rmap_locks, bool for_stack);
+unsigned long move_page_tables(struct pagetable_move_control *pmc);
 
 #ifdef CONFIG_UNACCEPTED_MEMORY
 void accept_page(struct page *page);