[PATCH RFC 09/13] mm/huge_memory: move memcg switch into the file split helper

[Kairui Song via B4 Relay]

From: Kairui Song <kasong@xxxxxxxxxxx>

The xarray node allocations in __folio_freeze_split_unmap_file() need
to be charged to the folio's memcg, so move the memcg switch from
__folio_split() into the helper.

The anon split helper and the after-split folio freeing perform no
chargeable allocations, so no memcg handling is left in __folio_split().
Rename its out_no_memcg label to out.

Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
 mm/huge_memory.c | 36 +++++++++++++++++++-----------------
 1 file changed, 19 insertions(+), 17 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f0ea6e5f53f7..ab2bb29748d3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4068,6 +4068,7 @@ static int __folio_freeze_split_unmap_file(struct folio *folio, unsigned int new
 	struct address_space *mapping = folio->mapping;
 	XA_STATE(xas, &mapping->i_pages, folio->index);
 	struct folio *end_folio = folio_next(folio);
+	struct mem_cgroup *memcg, *old_memcg;
 	struct folio *new_folio, *next;
 	int nr_shmem_dropped = 0;
 	unsigned int min_order;
@@ -4080,9 +4081,18 @@ static int __folio_freeze_split_unmap_file(struct folio *folio, unsigned int new
 	if (new_order < min_order)
 		return -EINVAL;
 
+	/*
+	 * Switch to folio's memcg as xarray node allocation can happen and
+	 * needs to charge to it.
+	 */
+	memcg = get_mem_cgroup_from_folio(folio);
+	old_memcg = set_active_memcg(memcg);
+
 	gfp = current_gfp_context(mapping_gfp_mask(mapping) & GFP_RECLAIM_MASK);
-	if (!filemap_release_folio(folio, gfp))
-		return -EBUSY;
+	if (!filemap_release_folio(folio, gfp)) {
+		ret = -EBUSY;
+		goto fail_free;
+	}
 
 	mapping_set_update(&xas, mapping);
 
@@ -4215,6 +4225,9 @@ static int __folio_freeze_split_unmap_file(struct folio *folio, unsigned int new
 	 */
 	i_mmap_unlock_read(mapping);
 fail_free:
+	/* Restore the previously active memcg */
+	set_active_memcg(old_memcg);
+	mem_cgroup_put(memcg);
 	xas_destroy(&xas);
 	return ret;
 }
@@ -4246,7 +4259,6 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 {
 	struct folio *end_folio = folio_next(folio);
 	bool is_anon = folio_test_anon(folio);
-	struct mem_cgroup *memcg, *old_memcg;
 	int old_order = folio_order(folio);
 	struct folio *new_folio, *next;
 	int ret;
@@ -4256,27 +4268,20 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 
 	if (folio != page_folio(split_at) || folio != page_folio(lock_at)) {
 		ret = -EINVAL;
-		goto out_no_memcg;
+		goto out;
 	}
 
 	if (new_order >= old_order) {
 		ret = -EINVAL;
-		goto out_no_memcg;
+		goto out;
 	}
 
 	ret = folio_check_splittable(folio, new_order, split_type);
 	if (ret) {
 		VM_WARN_ONCE(ret == -EINVAL, "Tried to split an unsplittable folio");
-		goto out_no_memcg;
+		goto out;
 	}
 
-	/*
-	 * switch to folio's memcg as xarray node allocation can happen and
-	 * needs to charge to it.
-	 */
-	memcg = get_mem_cgroup_from_folio(folio);
-	old_memcg = set_active_memcg(memcg);
-
 	if (is_anon)
 		ret = __folio_freeze_split_unmap_anon(folio, new_order, split_at, true,
 						      true, list, split_type);
@@ -4303,10 +4308,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		free_folio_and_swap_cache(new_folio);
 	}
 
-	/* restore to caller's old_memcg */
-	set_active_memcg(old_memcg);
-	mem_cgroup_put(memcg);
-out_no_memcg:
+out:
 	if (is_pmd_order(old_order))
 		count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
 	count_mthp_stat(old_order, !ret ? MTHP_STAT_SPLIT : MTHP_STAT_SPLIT_FAILED);

-- 
2.55.0