+ mm-huge_memory-use-folios-memcg-inside-__folio_split.patch added to mm-new branch

+ mm-huge_memory-use-folios-memcg-inside-__folio_split.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm/huge_memory: use folio's memcg inside __folio_split()
has been added to the -mm mm-new branch.  Its filename is
     mm-huge_memory-use-folios-memcg-inside-__folio_split.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-huge_memory-use-folios-memcg-inside-__folio_split.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Zi Yan <ziy@nvidia.com>
Subject: mm/huge_memory: use folio's memcg inside __folio_split()
Date: Thu, 30 Jul 2026 22:18:00 -0400

Patch series "Honor XA_FLAGS_ACCOUNT in xas_split_alloc() and charge to
folio's memcg", v2.

__GFP_ACCOUNT is needed for xarray node allocation accounting when
XA_FLAGS_ACCOUNT is set. Commit 7b785645e8f13 ("mm: fix page cache
convergence regression") fixed a workingset regression with it.
xas_split_alloc() does not have it and needs to be fixed.

In addition, based on Sashiko's review[1] and Johannes' confirmation[2], to
charge the right memcg, folio's memcg needs to be active during folio
split. Add that before adding __GFP_ACCOUNT.

There is no workingset convergence regression related to missing
__GFP_ACCOUNT in xas_split_alloc() and the impact to userspace should be
minor.


During a pagecache folio split, an xarray node allocation can happen and
needs to charge at folio's memcg instead of folio split invoker's memcg,
because for example folio split can happen during reclaim and reclaim's
active memcg might not be folio's memcg.  Switch to folio's memcg at the
beginning and switch back afterwards.

Link: https://lore.kernel.org/20260730-add-gfp_account-to-xas_split_alloc-v2-0-5a56c2361d85@nvidia.com
Link: https://lore.kernel.org/20260730-add-gfp_account-to-xas_split_alloc-v2-1-5a56c2361d85@nvidia.com
Link: https://sashiko.dev/#/patchset/20260727-add-gfp_account-to-xas_split_alloc-v1-1-9fae6bf64838%40nvidia.com?part=1 [1]
Link: https://lore.kernel.org/all/amtcBZ-_QVRgCd6b@cmpxchg.org/ [2]
Fixes: 6b24ca4a1a8d4 ("mm: Use multi-index entries in the page cache")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/huge_memory.c |   20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

--- a/mm/huge_memory.c~mm-huge_memory-use-folios-memcg-inside-__folio_split
+++ a/mm/huge_memory.c
@@ -4106,34 +4106,42 @@ static int __folio_split(struct folio *f
 	XA_STATE(xas, &folio->mapping->i_pages, folio->index);
 	struct folio *end_folio = folio_next(folio);
 	bool is_anon = folio_test_anon(folio);
+	struct mem_cgroup *memcg, *old_memcg;
 	struct address_space *mapping = NULL;
 	struct anon_vma *anon_vma = NULL;
 	int old_order = folio_order(folio);
 	struct folio *new_folio, *next;
 	int nr_shmem_dropped = 0;
 	enum ttu_flags ttu_flags = 0;
-	int ret;
 	pgoff_t end = 0;
+	int ret;
 
 	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
 	VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
 
 	if (folio != page_folio(split_at) || folio != page_folio(lock_at)) {
 		ret = -EINVAL;
-		goto out;
+		goto out_no_memcg;
 	}
 
 	if (new_order >= old_order) {
 		ret = -EINVAL;
-		goto out;
+		goto out_no_memcg;
 	}
 
 	ret = folio_check_splittable(folio, new_order, split_type);
 	if (ret) {
 		VM_WARN_ONCE(ret == -EINVAL, "Tried to split an unsplittable folio");
-		goto out;
+		goto out_no_memcg;
 	}
 
+	/*
+	 * 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) {
 		/*
 		 * The caller does not necessarily hold an mmap_lock that would
@@ -4276,6 +4284,10 @@ out_unlock:
 	if (mapping)
 		i_mmap_unlock_read(mapping);
 out:
+	/* restore to caller's old_memcg */
+	set_active_memcg(old_memcg);
+	mem_cgroup_put(memcg);
+out_no_memcg:
 	xas_destroy(&xas);
 	if (is_pmd_order(old_order))
 		count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
_

Patches currently in -mm which might be from ziy@nvidia.com are

mm-percpu-km-clear-page-private-before-free-them.patch
mm-compaction-stop-recording-free-page-order-in-page-private.patch
mm-huge_memory-add-page-private-check-back-in-__split_folio_to_order.patch
mm-page_alloc-make-sure-tail_page-private-is-zero-at-page-free-time.patch
mm-page_alloc-remove-set_page_private-in-prep_compound_tail.patch
mm-huge_memory-use-folios-memcg-inside-__folio_split.patch
xarray-honor-xa_flags_account-in-xas_split_alloc.patch