bcachefs: Ratelimiting for writeback IOs

Writeback throttling is a kernel config option and not always enabled.
When it's not enabled we need a fallback, to avoid unbounded memory
pinning and work item backlogs.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -191,6 +191,7 @@
 #include <linux/percpu-rwsem.h>
 #include <linux/rhashtable.h>
 #include <linux/rwsem.h>
+#include <linux/semaphore.h>
 #include <linux/seqlock.h>
 #include <linux/shrinker.h>
 #include <linux/srcu.h>
@@ -746,6 +747,7 @@ struct bch_fs {
 	struct rw_semaphore	gc_lock;
 
 	/* IO PATH */
+	struct semaphore	io_in_flight;
 	struct bio_set		bio_read;
 	struct bio_set		bio_read_split;
 	struct bio_set		bio_write;

fs/bcachefs/fs-io.c

@@ -997,6 +997,8 @@ static void bch2_writepage_io_done(struct closure *cl)
 	struct bio_vec *bvec;
 	unsigned i;
 
+	up(&io->op.c->io_in_flight);
+
 	if (io->op.error) {
 		set_bit(EI_INODE_ERROR, &io->inode->ei_flags);
 
@@ -1059,6 +1061,8 @@ static void bch2_writepage_do_io(struct bch_writepage_state *w)
 {
 	struct bch_writepage_io *io = w->io;
 
+	down(&io->op.c->io_in_flight);
+
 	w->io = NULL;
 	closure_call(&io->op.cl, bch2_write, NULL, &io->cl);
 	continue_at(&io->cl, bch2_writepage_io_done, NULL);

fs/bcachefs/super.c

@@ -717,6 +717,8 @@ static struct bch_fs *bch2_fs_alloc(struct bch_sb *sb, struct bch_opts opts)
 
 	seqcount_init(&c->usage_lock);
 
+	sema_init(&c->io_in_flight, 128);
+
 	c->copy_gc_enabled		= 1;
 	c->rebalance.enabled		= 1;
 	c->promote_whole_extents	= true;