Commit 2a4e7497 authored by Brian Foster's avatar Brian Foster Committed by Kent Overstreet
Browse files

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: default avatarBrian Foster <bfoster@redhat.com>
Signed-off-by: default avatarKent Overstreet <kent.overstreet@linux.dev>
parent 0e91d3a6

fs/bcachefs/fs-io-buffered.c

+16 −3
Original line number Diff line number Diff line
@@ -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 @@ static int __bch2_writepage(struct folio *folio, 


		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);