[RFCv3 1/3] iomap: Allocate iop in ->write_begin() early

["Ritesh Harjani (IBM)" <ritesh.list@gmail.com>]

Earlier when the folio is uptodate, we only allocate iop at writeback
time (in iomap_writepage_map()). This is ok until now, but when we are
going to add support for subpage size dirty bitmap tracking in iop, this
could cause some performance degradation. The reason is that if we don't
allocate iop during ->write_begin(), then we will never mark the
necessary dirty bits in ->write_end() call. And we will have to mark all
the bits as dirty at the writeback time, that could cause the same write
amplification and performance problems as it is now (w/o subpage dirty
bitmap tracking in iop).

However, for all the writes with (pos, len) which completely overlaps
the given folio, there is no need to allocate an iop during
->write_begin(). So skip those cases.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
 fs/iomap/buffered-io.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 356193e44cf0..c5b51ab1184e 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -535,11 +535,16 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
 	size_t from = offset_in_folio(folio, pos), to = from + len;
 	size_t poff, plen;
 
+	if (pos <= folio_pos(folio) &&
+	    pos + len >= folio_pos(folio) + folio_size(folio))
+		return 0;
+
+	iop = iomap_page_create(iter->inode, folio, iter->flags);
+
 	if (folio_test_uptodate(folio))
 		return 0;
 	folio_clear_error(folio);
 
-	iop = iomap_page_create(iter->inode, folio, iter->flags);
 	if ((iter->flags & IOMAP_NOWAIT) && !iop && nr_blocks > 1)
 		return -EAGAIN;
 
-- 
2.39.2