bcachefs: Nocow support

This adds support for nocow mode, where we do writes in-place when
possible. Patch components:

 - New boolean filesystem and inode option, nocow: note that when nocow
   is enabled, data checksumming and compression are implicitly disabled

 - To prevent in-place writes from racing with data moves
   (data_update.c) or bucket reuse (i.e. a bucket being reused and
   re-allocated while a nocow write is in flight, we have a new locking
   mechanism.

   Buckets can be locked for either data update or data move, using a
   fixed size hash table of two_state_shared locks. We don't have any
   chaining, meaning updates and moves to different buckets that hash to
   the same lock will wait unnecessarily - we'll want to watch for this
   becoming an issue.

 - The allocator path also needs to check for in-place writes in flight
   to a given bucket before giving it out: thus we add another counter
   to bucket_alloc_state so we can track this.

 - Fsync now may need to issue cache flushes to block devices instead of
   flushing the journal. We add a device bitmask to bch_inode_info,
   ei_devs_need_flush, which tracks devices that need to have flushes
   issued - note that this will lead to unnecessary flushes when other
   codepaths have already issued flushes, we may want to replace this with
   a sequence number.

 - New nocow write path: look up extents, and if they're writable write
   to them - otherwise fall back to the normal COW write path.

XXX: switch to sequence numbers instead of bitmask for devs needing
journal flush

XXX: ei_quota_lock being a mutex means bch2_nocow_write_done() needs to
run in process context - see if we can improve this

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 55 insertions, 0 deletions

diff --git a/fs/bcachefs/nocow_locking.h b/fs/bcachefs/nocow_locking.h
new file mode 100644
index 000000000000..2a7a9f44e88e
--- /dev/null
+++ b/fs/bcachefs/nocow_locking.h
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _BCACHEFS_NOCOW_LOCKING_H
+#define _BCACHEFS_NOCOW_LOCKING_H
+
+#include "bcachefs_format.h"
+#include "two_state_shared_lock.h"
+
+#include <linux/hash.h>
+
+#define BUCKET_NOCOW_LOCKS_BITS		10
+#define BUCKET_NOCOW_LOCKS		(1U << BUCKET_NOCOW_LOCKS_BITS)
+
+struct bucket_nocow_lock_table {
+	two_state_lock_t		l[BUCKET_NOCOW_LOCKS];
+};
+
+#define BUCKET_NOCOW_LOCK_UPDATE	(1 << 0)
+
+static inline two_state_lock_t *bucket_nocow_lock(struct bucket_nocow_lock_table *t,
+						  struct bpos bucket)
+{
+	u64 dev_bucket = bucket.inode << 56 | bucket.offset;
+	unsigned h = hash_64(dev_bucket, BUCKET_NOCOW_LOCKS_BITS);
+
+	return t->l + (h & (BUCKET_NOCOW_LOCKS - 1));
+}
+
+static inline bool bch2_bucket_nocow_is_locked(struct bucket_nocow_lock_table *t,
+					       struct bpos bucket)
+{
+	two_state_lock_t *l = bucket_nocow_lock(t, bucket);
+
+	return atomic_long_read(&l->v) != 0;
+}
+
+static inline void bch2_bucket_nocow_unlock(struct bucket_nocow_lock_table *t,
+					    struct bpos bucket, int flags)
+{
+	two_state_lock_t *l = bucket_nocow_lock(t, bucket);
+
+	bch2_two_state_unlock(l, flags & BUCKET_NOCOW_LOCK_UPDATE);
+}
+
+void __bch2_bucket_nocow_lock(struct bucket_nocow_lock_table *, two_state_lock_t *, int);
+
+static inline void bch2_bucket_nocow_lock(struct bucket_nocow_lock_table *t,
+					  struct bpos bucket, int flags)
+{
+	two_state_lock_t *l = bucket_nocow_lock(t, bucket);
+
+	if (!bch2_two_state_trylock(l, flags & BUCKET_NOCOW_LOCK_UPDATE))
+		__bch2_bucket_nocow_lock(t, l, flags);
+}
+
+#endif /* _BCACHEFS_NOCOW_LOCKING_H */