+ erofs-mm-pagemap-add-readahead_folio_last-to-avoid-folio-private.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: erofs: mm/pagemap: add readahead_folio_last() to avoid folio->private
has been added to the -mm mm-new branch.  Its filename is
     erofs-mm-pagemap-add-readahead_folio_last-to-avoid-folio-private.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/erofs-mm-pagemap-add-readahead_folio_last-to-avoid-folio-private.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: erofs: mm/pagemap: add readahead_folio_last() to avoid folio->private
Date: Sun, 13 Sep 2026 22:24:06 -0400

erofs needs to traverse readahead folios in reverse order to achieve
maximum performance by
1. reading all folios from readahead_folio();
2. storing the prior folio pointer in folio->private;
3. traverse from the last folio to the first one.

Add readahead_folio_last() to achieve the same function without using
folio->private.  __readahead_advance() helper shares readahead_control
adjustment code among __readahead_folio(), readahead_folio_last(), and
__readahead_batch() by checking new private member, _forward, of
readahead_control.

It prepares for a future commit that replaces PG_private checks with
!folio->private checks.  After switching the checks, erofs's use of
folio->private without bumping folio refcount can cause unexpected
outcomes, e.g., in filemap_release_folio(), try_to_free_buffers() becomes
reachable.

No functional change intended.

Assisted-by: LLM
Link: https://lore.kernel.org/20260913-remove-pg_private-v4-8-848550f7574e@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Yue Hu <zbestahu@gmail.com>
Cc: Jeffle Xu <jefflexu@linux.alibaba.com>
Cc: Sandeep Dhavale <dhavale@google.com>
Cc: Hongbo Li <hongbohbli@tencent.com>
Cc: Chunhai Guo <guochunhai@vivo.com>
Cc: Gao Xiang <xiang@kernel.org>
Cc: Chao Yu <chao@kernel.org>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
---

 fs/erofs/zdata.c        |   13 +-------
 include/linux/pagemap.h |   60 ++++++++++++++++++++++++++++++++------
 2 files changed, 55 insertions(+), 18 deletions(-)

--- a/fs/erofs/zdata.c~erofs-mm-pagemap-add-readahead_folio_last-to-avoid-folio-private
+++ a/fs/erofs/zdata.c
@@ -1898,21 +1898,14 @@ static void z_erofs_readahead(struct rea
 	struct inode *realinode = erofs_real_inode(sharedinode, &need_iput);
 	Z_EROFS_DEFINE_FRONTEND(f, realinode, sharedinode, readahead_pos(rac));
 	unsigned int nrpages = readahead_count(rac);
-	struct folio *head = NULL, *folio;
+	struct folio *folio;
 	int err;
 
 	trace_erofs_readahead(realinode, readahead_index(rac), nrpages, false);
 	z_erofs_pcluster_readmore(&f, rac, true);
-	while ((folio = readahead_folio(rac))) {
-		folio->private = head;
-		head = folio;
-	}
-
-	/* traverse in reverse order for best metadata I/O performance */
-	while (head) {
-		folio = head;
-		head = folio_get_private(folio);
 
+	/* traverse from last to first for best metadata I/O performance */
+	while ((folio = readahead_folio_last(rac))) {
 		err = z_erofs_scan_folio(&f, folio, true);
 		if (err && err != -EINTR)
 			erofs_err(realinode->i_sb, "readahead error at folio %lu @ nid %llu",
--- a/include/linux/pagemap.h~erofs-mm-pagemap-add-readahead_folio_last-to-avoid-folio-private
+++ a/include/linux/pagemap.h
@@ -1415,6 +1415,7 @@ struct readahead_control {
 	bool dropbehind;
 	bool _workingset;
 	unsigned long _pflags;
+	bool _forward;
 };
 
 #define DEFINE_READAHEAD(ractl, f, r, m, i)				\
@@ -1479,18 +1480,29 @@ void page_cache_async_readahead(struct a
 	page_cache_async_ra(&ractl, folio, req_count);
 }
 
+/*
+ * Adjust readahead_control to ensure next folio comes from
+ * [_index, _index + _nr_pages) afterwards and reset _batch_count.
+ */
+static inline void __readahead_advance(struct readahead_control *rac)
+{
+	if (rac->_forward)
+		rac->_index += rac->_batch_count;
+
+	rac->_nr_pages -= rac->_batch_count;
+	rac->_batch_count = 0;
+}
+
 static inline struct folio *__readahead_folio(struct readahead_control *ractl)
 {
 	struct folio *folio;
 
 	BUG_ON(ractl->_batch_count > ractl->_nr_pages);
-	ractl->_nr_pages -= ractl->_batch_count;
-	ractl->_index += ractl->_batch_count;
+	__readahead_advance(ractl);
+	ractl->_forward = true;
 
-	if (!ractl->_nr_pages) {
-		ractl->_batch_count = 0;
+	if (!ractl->_nr_pages)
 		return NULL;
-	}
 
 	folio = xa_load(&ractl->mapping->i_pages, ractl->_index);
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
@@ -1516,6 +1528,39 @@ static inline struct folio *readahead_fo
 	return folio;
 }
 
+/**
+ * readahead_folio_last - Get the next folio to read, from the tail.
+ * @ractl: The current readahead request.
+ *
+ * Like readahead_folio(), but walks the range back-to-front. The folio is
+ * returned locked with its refcount dropped; the caller unlocks it once I/O
+ * completes. Compound folios are returned once, at their head index.
+ *
+ * Context: The folio is locked.
+ * Return: A pointer to the next folio, or %NULL when done.
+ */
+static inline struct folio *readahead_folio_last(struct readahead_control *ractl)
+{
+	struct folio *folio;
+
+	/* Drop the previously returned batch from the remaining range. */
+	__readahead_advance(ractl);
+	ractl->_forward = false;
+
+	if (!ractl->_nr_pages)
+		return NULL;
+
+	/* xa_load() follows sibling entries, so a tail index returns the head */
+	folio = xa_load(&ractl->mapping->i_pages,
+			ractl->_index + ractl->_nr_pages - 1);
+	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
+
+	ractl->_batch_count = folio_nr_pages(folio);
+
+	folio_put(folio);
+	return folio;
+}
+
 static inline unsigned int __readahead_batch(struct readahead_control *rac,
 		struct page **array, unsigned int array_sz)
 {
@@ -1524,9 +1569,8 @@ static inline unsigned int __readahead_b
 	struct folio *folio;
 
 	BUG_ON(rac->_batch_count > rac->_nr_pages);
-	rac->_nr_pages -= rac->_batch_count;
-	rac->_index += rac->_batch_count;
-	rac->_batch_count = 0;
+	__readahead_advance(rac);
+	rac->_forward = true;
 
 	xas_set(&xas, rac->_index);
 	rcu_read_lock();
_

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

mm-zsmalloc-replace-pg_private-with-pointer-comparison.patch
perf-ring_buffer-stop-using-pg_private-as-aux-page-high-order-marker.patch
xen-grant-table-stop-setting-pg_private-on-pages-for-grant-mapping.patch
fscrypt-stop-setting-pg_private-on-bounce-page.patch
mm-hugetlb-use-direct-assignment-instead-of-folio_change_private.patch
f2fs-stop-using-pg_private.patch
f2fs-convert-the-private-flag-helpers-to-folio-only.patch
erofs-mm-pagemap-add-readahead_folio_last-to-avoid-folio-private.patch
erofs-use-folio_attach-detach_private-instead-of-direct-assignment.patch
mm-page-flags-check-page-folio-private-instead-of-pg_private.patch
treewide-remove-folio_set-clear_private-usage.patch
ceph-replace-pageprivate-with-page_private.patch
md-md-bitmap-replace-pageprivate-with-page_private.patch
buffer-replace-page_buffer-with-page_private-and-delete-it.patch
treewide-remove-pageprivate-and-pg_private-from-comments-and-docs.patch
mm-page-flags-remove-pg_private.patch
usb-gadgetfs-do-not-warn-about-excessively-large-memory-allocations.patch