[PATCH] iomap: replace folio_batch allocation with stack allocation

[Brian Foster <bfoster@redhat.com>]

Zhang Yi points out that the dynamic folio_batch allocation in
iomap_fill_dirty_folios() is problematic for the ext4 on iomap work
that is under development because it doesn't sufficiently handle the
allocation failure case (by allowing a retry, for example).

The dynamic allocation was initially added for simplicity and to
help indicate whether the batch was used or not by the calling fs.
To address this issue, put the batch on the stack of
iomap_zero_range() and use a flag to control whether the batch
should be used in the iomap folio lookup path. This keeps things
simple and eliminates the concern for ext4 on iomap.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Acked-by: Dave Chinner <dchinner@redhat.com>
---

Hi all,

This is patch 1 from the previously posted RFC series here [1]. This was
initially motivated by the ext4-on-iomap WIP, but the folio batch
allocation has also since been called out separately here [2]. The
broader series at [1] still needs work, but since this is a straight
cleanup I'm posting it as such for standalone consideration.

Thoughts, reviews, flames appreciated.

Brian

[1] https://lore.kernel.org/linux-fsdevel/20251016190303.53881-1-bfoster@redhat.com/
[2] https://lore.kernel.org/linux-xfs/aQu8B63pEAzGRAkj@dread.disaster.area/

 fs/iomap/buffered-io.c | 45 ++++++++++++++++++++++++++++--------------
 fs/iomap/iter.c        |  6 +++---
 fs/xfs/xfs_iomap.c     | 11 ++++++-----
 include/linux/iomap.h  |  8 ++++++--
 4 files changed, 45 insertions(+), 25 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 9b0b9cf7caa7..618f7f02f1bd 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -772,7 +772,7 @@ static struct folio *__iomap_get_folio(struct iomap_iter *iter,
 	if (!mapping_large_folio_support(iter->inode->i_mapping))
 		len = min_t(size_t, len, PAGE_SIZE - offset_in_page(pos));
 
-	if (iter->fbatch) {
+	if (iter->iomap.flags & IOMAP_F_FOLIO_BATCH) {
 		struct folio *folio = folio_batch_next(iter->fbatch);
 
 		if (!folio)
@@ -869,7 +869,7 @@ static int iomap_write_begin(struct iomap_iter *iter,
 	 * process so return and let the caller iterate and refill the batch.
 	 */
 	if (!folio) {
-		WARN_ON_ONCE(!iter->fbatch);
+		WARN_ON_ONCE(!(iter->iomap.flags & IOMAP_F_FOLIO_BATCH));
 		return 0;
 	}
 
@@ -1483,23 +1483,34 @@ static int iomap_zero_iter(struct iomap_iter *iter, bool *did_zero,
 	return status;
 }
 
-loff_t
+/**
+ * iomap_fill_dirty_folios - fill a folio batch with dirty folios
+ * @iter: Iteration structure
+ * @start: Start offset of range. Updated based on lookup progress.
+ * @end: End offset of range
+ *
+ * Returns the associated control flag if the folio batch is available and the
+ * lookup performed. The caller is responsible to set the flag on the associated
+ * iomap.
+ */
+unsigned int
 iomap_fill_dirty_folios(
 	struct iomap_iter	*iter,
-	loff_t			offset,
-	loff_t			length)
+	loff_t			*start,
+	loff_t			end)
 {
 	struct address_space	*mapping = iter->inode->i_mapping;
-	pgoff_t			start = offset >> PAGE_SHIFT;
-	pgoff_t			end = (offset + length - 1) >> PAGE_SHIFT;
+	pgoff_t			pstart = *start >> PAGE_SHIFT;
+	pgoff_t			pend = (end - 1) >> PAGE_SHIFT;
 
-	iter->fbatch = kmalloc(sizeof(struct folio_batch), GFP_KERNEL);
-	if (!iter->fbatch)
-		return offset + length;
-	folio_batch_init(iter->fbatch);
+	if (!iter->fbatch) {
+		*start = end;
+		return 0;
+	}
 
-	filemap_get_folios_dirty(mapping, &start, end, iter->fbatch);
-	return (start << PAGE_SHIFT);
+	filemap_get_folios_dirty(mapping, &pstart, pend, iter->fbatch);
+	*start = (pstart << PAGE_SHIFT);
+	return IOMAP_F_FOLIO_BATCH;
 }
 EXPORT_SYMBOL_GPL(iomap_fill_dirty_folios);
 
@@ -1508,17 +1519,21 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
 		const struct iomap_ops *ops,
 		const struct iomap_write_ops *write_ops, void *private)
 {
+	struct folio_batch fbatch;
 	struct iomap_iter iter = {
 		.inode		= inode,
 		.pos		= pos,
 		.len		= len,
 		.flags		= IOMAP_ZERO,
 		.private	= private,
+		.fbatch		= &fbatch,
 	};
 	struct address_space *mapping = inode->i_mapping;
 	int ret;
 	bool range_dirty;
 
+	folio_batch_init(&fbatch);
+
 	/*
 	 * To avoid an unconditional flush, check pagecache state and only flush
 	 * if dirty and the fs returns a mapping that might convert on
@@ -1529,11 +1544,11 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
 	while ((ret = iomap_iter(&iter, ops)) > 0) {
 		const struct iomap *srcmap = iomap_iter_srcmap(&iter);
 
-		if (WARN_ON_ONCE(iter.fbatch &&
+		if (WARN_ON_ONCE((iter.iomap.flags & IOMAP_F_FOLIO_BATCH) &&
 				 srcmap->type != IOMAP_UNWRITTEN))
 			return -EIO;
 
-		if (!iter.fbatch &&
+		if (!(iter.iomap.flags & IOMAP_F_FOLIO_BATCH) &&
 		    (srcmap->type == IOMAP_HOLE ||
 		     srcmap->type == IOMAP_UNWRITTEN)) {
 			s64 status;
diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
index 8692e5e41c6d..c04796f6e57f 100644
--- a/fs/iomap/iter.c
+++ b/fs/iomap/iter.c
@@ -8,10 +8,10 @@
 
 static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
 {
-	if (iter->fbatch) {
+	if (iter->iomap.flags & IOMAP_F_FOLIO_BATCH) {
 		folio_batch_release(iter->fbatch);
-		kfree(iter->fbatch);
-		iter->fbatch = NULL;
+		folio_batch_reinit(iter->fbatch);
+		iter->iomap.flags &= ~IOMAP_F_FOLIO_BATCH;
 	}
 
 	iter->status = 0;
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 04f39ea15898..639a85bd260a 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1831,7 +1831,6 @@ xfs_buffered_write_iomap_begin(
 	 */
 	if (flags & IOMAP_ZERO) {
 		xfs_fileoff_t eof_fsb = XFS_B_TO_FSB(mp, XFS_ISIZE(ip));
-		u64 end;
 
 		if (isnullstartblock(imap.br_startblock) &&
 		    offset_fsb >= eof_fsb)
@@ -1851,12 +1850,14 @@ xfs_buffered_write_iomap_begin(
 		 */
 		if (imap.br_state == XFS_EXT_UNWRITTEN &&
 		    offset_fsb < eof_fsb) {
-			loff_t len = min(count,
-					 XFS_FSB_TO_B(mp, imap.br_blockcount));
+			loff_t foffset = offset, fend;
 
-			end = iomap_fill_dirty_folios(iter, offset, len);
+			fend = offset +
+			       min(count, XFS_FSB_TO_B(mp, imap.br_blockcount));
+			iomap_flags |= iomap_fill_dirty_folios(iter, &foffset,
+							       fend);
 			end_fsb = min_t(xfs_fileoff_t, end_fsb,
-					XFS_B_TO_FSB(mp, end));
+					XFS_B_TO_FSB(mp, foffset));
 		}
 
 		xfs_trim_extent(&imap, offset_fsb, end_fsb - offset_fsb);
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 8b1ac08c7474..fc3aa944d22f 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -88,6 +88,9 @@ struct vm_fault;
 /*
  * Flags set by the core iomap code during operations:
  *
+ * IOMAP_F_FOLIO_BATCH indicates that the folio batch mechanism is active
+ * for this operation, set by iomap_fill_dirty_folios().
+ *
  * IOMAP_F_SIZE_CHANGED indicates to the iomap_end method that the file size
  * has changed as the result of this write operation.
  *
@@ -95,6 +98,7 @@ struct vm_fault;
  * range it covers needs to be remapped by the high level before the operation
  * can proceed.
  */
+#define IOMAP_F_FOLIO_BATCH	(1U << 13)
 #define IOMAP_F_SIZE_CHANGED	(1U << 14)
 #define IOMAP_F_STALE		(1U << 15)
 
@@ -352,8 +356,8 @@ bool iomap_dirty_folio(struct address_space *mapping, struct folio *folio);
 int iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
 		const struct iomap_ops *ops,
 		const struct iomap_write_ops *write_ops);
-loff_t iomap_fill_dirty_folios(struct iomap_iter *iter, loff_t offset,
-		loff_t length);
+unsigned int iomap_fill_dirty_folios(struct iomap_iter *iter, loff_t *start,
+		loff_t end);
 int iomap_zero_range(struct inode *inode, loff_t pos, loff_t len,
 		bool *did_zero, const struct iomap_ops *ops,
 		const struct iomap_write_ops *write_ops, void *private);
-- 
2.51.1