[PATCH v2 03/12] mm: hugetlb_vmemmap: simplify vmemmap head page handling

[Muchun Song <songmuchun@bytedance.com>]

The new head page is already tracked on the vmemmap page list before
remapping starts.  Passing it separately gives the remap walker two
ways to refer to the same page and unnecessarily complicates its
interface.

Use the page list as the single handoff mechanism for the head page,
removing the redundant state from the walker.

No functional change intended.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Qi Zheng <qi.zheng@linux.dev>
---
v2:
- Remove outdated head-page wording from vmemmap_remap_free() kernel-doc
  (suggested by Lance Yang)
- Document the vmemmap page list contract (suggested by Qi Zheng)
- Collect Reviewed-by from Lance Yang and Qi Zheng
---
 mm/hugetlb_vmemmap.c | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 63e899cfaac3..0e5b2b182d1b 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -25,10 +25,9 @@
  *
  * @remap_pte:		called for each lowest-level entry (PTE).
  * @nr_walked:		the number of walked pte.
- * @vmemmap_head:	the page to be installed as first in the vmemmap range
  * @vmemmap_tail:	the page to be installed as non-first in the vmemmap range
- * @vmemmap_pages:	the list head of the vmemmap pages that can be freed
- *			or is mapped from.
+ * @vmemmap_pages:	list supplying replacement pages for the remap and
+ *			receiving displaced backing pages for the caller to free.
  * @flags:		used to modify behavior in vmemmap page table walking
  *			operations.
  */
@@ -37,7 +36,6 @@ struct vmemmap_remap_walk {
 					     struct vmemmap_remap_walk *walk);
 
 	unsigned long		nr_walked;
-	struct page		*vmemmap_head;
 	struct page		*vmemmap_tail;
 	struct list_head	*vmemmap_pages;
 
@@ -208,10 +206,13 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
 	pte_t entry;
 
 	/* Remapping the head page requires r/w */
-	if (unlikely(walk->nr_walked == 0 && walk->vmemmap_head)) {
+	if (unlikely(walk->nr_walked == 0)) {
+		struct page *head;
+
 		VM_WARN_ON_ONCE(!PageHead((const struct page *)addr));
 
-		list_del(&walk->vmemmap_head->lru);
+		head = list_first_entry(walk->vmemmap_pages, struct page, lru);
+		list_del(&head->lru);
 
 		/*
 		 * Makes sure that preceding stores to the page contents from
@@ -220,7 +221,7 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
 		 */
 		smp_wmb();
 
-		entry = mk_pte(walk->vmemmap_head, PAGE_KERNEL);
+		entry = mk_pte(head, PAGE_KERNEL);
 	} else {
 		VM_WARN_ON_ONCE(!PageTail((const struct page *)addr));
 
@@ -284,23 +285,20 @@ static int vmemmap_remap_split(unsigned long start, unsigned long end)
 }
 
 /**
- * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
- *			to use @vmemmap_head/tail, then free vmemmap which
- *			the range are mapped to.
+ * vmemmap_remap_free - remap a vmemmap range and collect old backing pages
  * @start:	start address of the vmemmap virtual address range that we want
  *		to remap.
  * @end:	end address of the vmemmap virtual address range that we want to
  *		remap.
- * @vmemmap_head: the page to be installed as first in the vmemmap range
  * @vmemmap_tail: the page to be installed as non-first in the vmemmap range
- * @vmemmap_pages: list to deposit vmemmap pages to be freed.  It is callers
- *		responsibility to free pages.
+ * @vmemmap_pages: input/output list whose first entry supplies the new backing
+ *		page for @start; replaced backing pages are added to the list for
+ *		the caller to free.
  * @flags:	modifications to vmemmap_remap_walk flags
  *
  * Return: %0 on success, negative error code otherwise.
  */
 static int vmemmap_remap_free(unsigned long start, unsigned long end,
-			      struct page *vmemmap_head,
 			      struct page *vmemmap_tail,
 			      struct list_head *vmemmap_pages,
 			      unsigned long flags)
@@ -308,7 +306,6 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
 	int ret;
 	struct vmemmap_remap_walk walk = {
 		.remap_pte	= vmemmap_remap_pte,
-		.vmemmap_head	= vmemmap_head,
 		.vmemmap_tail	= vmemmap_tail,
 		.vmemmap_pages	= vmemmap_pages,
 		.flags		= flags,
@@ -549,8 +546,7 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
 	 * they can be freed by the caller.
 	 */
 	ret = vmemmap_remap_free(vmemmap_start, vmemmap_end,
-				 vmemmap_head, vmemmap_tail,
-				 vmemmap_pages, flags);
+				 vmemmap_tail, vmemmap_pages, flags);
 out:
 	if (ret)
 		folio_clear_hugetlb_vmemmap_optimized(folio);
-- 
2.54.0