iomap: rework iomap_write_begin() to return folio offset and length

iomap_write_begin() returns a folio based on current pos and
remaining length in the iter, and each caller then trims the
pos/length to the given folio. Clean this up a bit and let
iomap_write_begin() return the trimmed range along with the folio.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/20250506134118.911396-7-bfoster@redhat.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 19 insertions, 14 deletions

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 86482b848259..572446635c9a 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -679,11 +679,12 @@ static int iomap_read_folio_sync(loff_t block_start, struct folio *folio,
 	return submit_bio_wait(&bio);
 }
 
-static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
-		size_t len, struct folio *folio)
+static int __iomap_write_begin(const struct iomap_iter *iter, size_t len,
+		struct folio *folio)
 {
 	const struct iomap *srcmap = iomap_iter_srcmap(iter);
 	struct iomap_folio_state *ifs;
+	loff_t pos = iter->pos;
 	loff_t block_size = i_blocksize(iter->inode);
 	loff_t block_start = round_down(pos, block_size);
 	loff_t block_end = round_up(pos + len, block_size);
@@ -794,15 +795,22 @@ static int iomap_write_begin_inline(const struct iomap_iter *iter,
 	return iomap_read_inline_data(iter, folio);
 }
 
-static int iomap_write_begin(struct iomap_iter *iter, size_t len,
-		struct folio **foliop)
+/*
+ * Grab and prepare a folio for write based on iter state. Returns the folio,
+ * offset, and length. Callers can optionally pass a max length *plen,
+ * otherwise init to zero.
+ */
+static int iomap_write_begin(struct iomap_iter *iter, struct folio **foliop,
+		size_t *poffset, u64 *plen)
 {
 	const struct iomap_folio_ops *folio_ops = iter->iomap.folio_ops;
 	const struct iomap *srcmap = iomap_iter_srcmap(iter);
 	loff_t pos = iter->pos;
+	u64 len = min_t(u64, SIZE_MAX, iomap_length(iter));
 	struct folio *folio;
 	int status = 0;
 
+	len = min_not_zero(len, *plen);
 	BUG_ON(pos + len > iter->iomap.offset + iter->iomap.length);
 	if (srcmap != &iter->iomap)
 		BUG_ON(pos + len > srcmap->offset + srcmap->length);
@@ -834,20 +842,20 @@ static int iomap_write_begin(struct iomap_iter *iter, size_t len,
 		}
 	}
 
-	if (pos + len > folio_pos(folio) + folio_size(folio))
-		len = folio_pos(folio) + folio_size(folio) - pos;
+	pos = iomap_trim_folio_range(iter, folio, poffset, &len);
 
 	if (srcmap->type == IOMAP_INLINE)
 		status = iomap_write_begin_inline(iter, folio);
 	else if (srcmap->flags & IOMAP_F_BUFFER_HEAD)
 		status = __block_write_begin_int(folio, pos, len, NULL, srcmap);
 	else
-		status = __iomap_write_begin(iter, pos, len, folio);
+		status = __iomap_write_begin(iter, len, folio);
 
 	if (unlikely(status))
 		goto out_unlock;
 
 	*foliop = folio;
+	*plen = len;
 	return 0;
 
 out_unlock:
@@ -968,7 +976,7 @@ retry:
 			break;
 		}
 
-		status = iomap_write_begin(iter, bytes, &folio);
+		status = iomap_write_begin(iter, &folio, &offset, &bytes);
 		if (unlikely(status)) {
 			iomap_write_failed(iter->inode, iter->pos, bytes);
 			break;
@@ -976,7 +984,7 @@ retry:
 		if (iter->iomap.flags & IOMAP_F_STALE)
 			break;
 
-		pos = iomap_trim_folio_range(iter, folio, &offset, &bytes);
+		pos = iter->pos;
 
 		if (mapping_writably_mapped(mapping))
 			flush_dcache_folio(folio);
@@ -1296,14 +1304,12 @@ static int iomap_unshare_iter(struct iomap_iter *iter)
 		bool ret;
 
 		bytes = min_t(u64, SIZE_MAX, bytes);
-		status = iomap_write_begin(iter, bytes, &folio);
+		status = iomap_write_begin(iter, &folio, &offset, &bytes);
 		if (unlikely(status))
 			return status;
 		if (iomap->flags & IOMAP_F_STALE)
 			break;
 
-		iomap_trim_folio_range(iter, folio, &offset, &bytes);
-
 		ret = iomap_write_end(iter, bytes, bytes, folio);
 		__iomap_put_folio(iter, bytes, folio);
 		if (WARN_ON_ONCE(!ret))
@@ -1368,7 +1374,7 @@ static int iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
 		bool ret;
 
 		bytes = min_t(u64, SIZE_MAX, bytes);
-		status = iomap_write_begin(iter, bytes, &folio);
+		status = iomap_write_begin(iter, &folio, &offset, &bytes);
 		if (status)
 			return status;
 		if (iter->iomap.flags & IOMAP_F_STALE)
@@ -1377,7 +1383,6 @@ static int iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
 		/* warn about zeroing folios beyond eof that won't write back */
 		WARN_ON_ONCE(folio_pos(folio) > iter->inode->i_size);
 
-		iomap_trim_folio_range(iter, folio, &offset, &bytes);
 		folio_zero_range(folio, offset, bytes);
 		folio_mark_accessed(folio);