From 2a4e7497604b20b19b6d9dbd109c42900892d7c9 Mon Sep 17 00:00:00 2001 From: Brian Foster Date: Fri, 3 Nov 2023 09:09:36 -0400 Subject: [PATCH] bcachefs: allow writeback to fill bio completely The bcachefs folio writeback code includes a bio full check as well as a fixed size check to determine when to split off and submit writeback I/O. The inclusive check of the latter against the limit means that writeback can submit slightly prematurely. This is not a functional problem, but results in unnecessarily split I/Os and extent merging. This can be observed with a buffered write sized exactly to the current maximum value (1MB) and with key_merging_disabled=1. The latter prevents the merge from the second write such that a subsequent check of the extent list shows a 1020k extent followed by a contiguous 4k extent. The purpose for the fixed size check is also undocumented and somewhat obscure. Lift this check into a new helper that wraps the bio check, fix the comparison logic, and add a comment to document the purpose and how we might improve on this in the future. Signed-off-by: Brian Foster Signed-off-by: Kent Overstreet --- fs/bcachefs/fs-io-buffered.c | 19 ++++++++++++++++--- 1 file changed, 16 insertions(+), 3 deletions(-) diff --git a/fs/bcachefs/fs-io-buffered.c b/fs/bcachefs/fs-io-buffered.c index 58ccc7b91ac7..52f0e7acda3d 100644 --- a/fs/bcachefs/fs-io-buffered.c +++ b/fs/bcachefs/fs-io-buffered.c @@ -389,6 +389,21 @@ static inline struct bch_writepage_state bch_writepage_state_init(struct bch_fs return ret; } +/* + * Determine when a writepage io is full. We have to limit writepage bios to a + * single page per bvec (i.e. 1MB with 4k pages) because that is the limit to + * what the bounce path in bch2_write_extent() can handle. In theory we could + * loosen this restriction for non-bounce I/O, but we don't have that context + * here. Ideally, we can up this limit and make it configurable in the future + * when the bounce path can be enhanced to accommodate larger source bios. + */ +static inline bool bch_io_full(struct bch_writepage_io *io, unsigned len) +{ + struct bio *bio = &io->op.wbio.bio; + return bio_full(bio, len) || + (bio->bi_iter.bi_size + len > BIO_MAX_VECS * PAGE_SIZE); +} + static void bch2_writepage_io_done(struct bch_write_op *op) { struct bch_writepage_io *io = @@ -606,9 +621,7 @@ do_io: if (w->io && (w->io->op.res.nr_replicas != nr_replicas_this_write || - bio_full(&w->io->op.wbio.bio, sectors << 9) || - w->io->op.wbio.bio.bi_iter.bi_size + (sectors << 9) >= - (BIO_MAX_VECS * PAGE_SIZE) || + bch_io_full(w->io, sectors << 9) || bio_end_sector(&w->io->op.wbio.bio) != sector)) bch2_writepage_do_io(w);