Message-ID: <20120523034546.GB607@dhcp-172-18-216-138.mtv.corp.google.com>
Date:	Tue, 22 May 2012 23:45:49 -0400
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Tejun Heo <tejun@...gle.com>
Cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, agk@...hat.com
Subject: Re: [Bcache v13 11/16] bcache: Core btree code

On Tue, May 22, 2012 at 03:12:59PM -0700, Tejun Heo wrote:
> Hello, Kent.
> 
> On Wed, May 09, 2012 at 11:10:48PM -0400, Kent Overstreet wrote:
> > +#define BKEY_PADDED(key)					\
> > +	union { struct bkey key; uint64_t key ## _pad[8]; }
> 
> What does the '8' mean?  And why does struct bbio use 3 instead?  Does
> this have to be a macro?  Can't we have struct bkey_padded (or
> whatever)?

Struct bbio uses 3 because it only ever carries around a single pointer
(i.e. the pointer it's doing io to/from). I'll document that.

The 8 here is kind of arbitrary, it should be a constant defined
somewhere - it's just a nice round number so it doesn't overflow, means
it can fit 6 pointers.

It's a macro because it's used for embedding them in various structs -
that way the pad can be in an anonymous union. Doesn't work the way
you'd want for bkey_paddeds that are declared on the stack - I never
found a solution I liked.

> > +struct io {
> > +	/* Used to track sequential IO so it can be skipped */
> > +	struct hlist_node	hash;
> > +	struct list_head	lru;
> > +
> > +	unsigned long		jiffies;
> > +	unsigned		sequential;
> > +	sector_t		last;
> > +};
> 
> I don't think bcache can have struct io. :P

Is your complaint that there's a struct io somewhere else, or just the
potential namespace clash in the future? Regardless, I can change it.

> 
> > +struct dirty_io {
> > +	struct closure		cl;
> > +	struct cached_dev	*d;
> > +	struct bio		bio;
> > +};
> > +
> > +struct dirty {
> > +	struct rb_node		node;
> > +	BKEY_PADDED(key);
> > +	struct dirty_io		*io;
> > +};
> ...
> > +struct cache {
> 
> Nor these and so on.

You want me to prefix all my struct names with bcache_? Blech.

> 
> > +/* Bkey fields: all units are in sectors */
> > +
> > +#define KEY_FIELD(name, field, offset, size)				\
> > +	BITMASK(name, struct bkey, field, offset, size)
> > +
> > +#define PTR_FIELD(name, offset, size)					\
> > +	static inline uint64_t name(const struct bkey *k, unsigned i)	\
> > +	{ return (k->ptr[i] >> offset) & ~(((uint64_t) ~0) << size); }	\
> > +									\
> > +	static inline void SET_##name(struct bkey *k, unsigned i, uint64_t v)\
> > +	{								\
> > +		k->ptr[i] &= ~(~((uint64_t) ~0 << size) << offset);	\
> > +		k->ptr[i] |= v << offset;				\
> > +	}
> > +
> > +KEY_FIELD(KEY_PTRS,	header, 60, 3)
> > +KEY_FIELD(HEADER_SIZE,	header, 58, 2)
> > +KEY_FIELD(KEY_CSUM,	header, 56, 2)
> > +KEY_FIELD(KEY_PINNED,	header, 55, 1)
> > +KEY_FIELD(KEY_DIRTY,	header, 36, 1)
> > +
> > +KEY_FIELD(KEY_SIZE,	header, 20, 16)
> > +KEY_FIELD(KEY_DEV,	header, 0,  20)
> > +
> > +KEY_FIELD(KEY_SECTOR,	key,	16, 47)
> > +KEY_FIELD(KEY_SNAPSHOT,	key,	0,  16)
> > +
> > +PTR_FIELD(PTR_DEV,		51, 12)
> > +PTR_FIELD(PTR_OFFSET,		8,  43)
> > +PTR_FIELD(PTR_GEN,		0,  8)
> 
> So, apart from the the macros, key is 64bit containing the backend
> device and extent offset and size with the ptr fields somehow point to
> cache.  Am I understanding it correctly?  If right, I'm *tiny* bit
> apprehensive about using only 43bits for offset.  While the block 9
> bits means 52bit addressing, the 9 bit block size is now there mostly
> to work as buffer between memory bitness growth and storage device
> size growth so that we have 9 bit buffer as storage device reaches
> ulong addressing limit.  Probably those days are far out enough.

You're exactly right. I had similar thoughts about the offset size,
but... it'll last until we have 8 exabyte cache devices, and I can't
make it bigger without making PTR_DEV smaller or making the whole
pointer 16 bytes.

Shouldn't be that hard to make a version of the on disk format with 16
byte pointers that's backwards compatible though, when it becomes
necessary. And we can detect it at format time.

> > +void btree_read_done(struct closure *cl)
> > +{
> > +	struct btree *b = container_of(cl, struct btree, io.cl);
> > +	struct bset *i = b->sets[0].data;
> > +	struct btree_iter *iter = b->c->fill_iter;
> > +	const char *err = "bad btree header";
> > +	BUG_ON(b->nsets || b->written);
> > +
> > +	bbio_free(b->bio, b->c);
> > +	b->bio = NULL;
> > +
> > +	mutex_lock(&b->c->fill_lock);
> > +	iter->used = 0;
> > +
> > +	if (btree_node_io_error(b) ||
> > +	    !i->seq)
> > +		goto err;
> > +
> > +	for (;
> > +	     b->written < btree_blocks(b) && i->seq == b->sets[0].data->seq;
> > +	     i = write_block(b)) {
> > +		err = "unsupported bset version";
> > +		if (i->version > BCACHE_BSET_VERSION)
> > +			goto err;
> > +
> > +		err = "bad btree header";
> > +		if (b->written + set_blocks(i, b->c) > btree_blocks(b))
> > +			goto err;
> > +
> > +		err = "bad magic";
> > +		if (i->magic != bset_magic(b->c))
> > +			goto err;
> > +
> > +		err = "bad checksum";
> > +		switch (i->version) {
> > +		case 0:
> > +			if (i->csum != csum_set(i))
> > +				goto err;
> > +			break;
> > +		case BCACHE_BSET_VERSION:
> > +			if (i->csum != btree_csum_set(b, i))
> > +				goto err;
> > +			break;
> > +		}
> > +
> > +		err = "empty set";
> > +		if (i != b->sets[0].data && !i->keys)
> > +			goto err;
> > +
> > +		btree_iter_push(iter, i->start, end(i));
> > +
> > +		b->written += set_blocks(i, b->c);
> > +	}
> > +
> > +	err = "corrupted btree";
> > +	for (i = write_block(b);
> > +	     index(i, b) < btree_blocks(b);
> > +	     i = ((void *) i) + block_bytes(b->c))
> > +		if (i->seq == b->sets[0].data->seq)
> > +			goto err;
> > +
> > +	btree_sort_and_fix_extents(b, iter);
> > +
> > +	i = b->sets[0].data;
> > +	err = "short btree key";
> > +	if (b->sets[0].size &&
> > +	    bkey_cmp(&b->key, &b->sets[0].end) < 0)
> > +		goto err;
> > +
> > +	if (b->written < btree_blocks(b))
> > +		bset_init_next(b);
> > +
> > +	if (0) {
> > +err:		set_btree_node_io_error(b);
> > +		cache_set_error(b->c, "%s at bucket %lu, block %zu, %u keys",
> > +				err, PTR_BUCKET_NR(b->c, &b->key, 0),
> > +				index(i, b), i->keys);
> > +	}
> 
> Please don't do that.  Just define out: label, move error specific
> path to the end of the function and jump to out at the end of that.

Heh. Given that you're the second person to complain about that, I'll
change it.

> > +
> > +	mutex_unlock(&b->c->fill_lock);
> > +
> > +	spin_lock(&b->c->btree_read_time_lock);
> > +	time_stats_update(&b->c->btree_read_time, b->io_start_time);
> > +	spin_unlock(&b->c->btree_read_time_lock);
> > +
> > +	smp_wmb(); /* read_done is our write lock */
> > +	set_btree_node_read_done(b);
> > +
> > +	closure_return(cl);
> > +}
> > +
> > +static void btree_read_resubmit(struct closure *cl)
> > +{
> > +	struct btree *b = container_of(cl, struct btree, io.cl);
> > +
> > +	submit_bbio_split(b->bio, b->c, &b->key, 0);
> > +	continue_at(&b->io.cl, btree_read_done, system_wq);
> > +}
> 
> I suppose submit_bbio_split() can't fail here somehow unlike from
> btree_read() path?  If so, please add a comment to explain and maybe
> WARN_ON_ONCE() on failure.  Subtlety to comment ratio is *way* off.

Yes - because this is the resubmit patch, i.e. runs out of workqueue
context if the one in btree_read() failed, which is only possible when
it's running under generic_make_request() (because then we don't use the
mempool to avoid deadlock).

If the code for making generic_make_request() handle arbitrary sized
bios goes in, this code will be deleted :)

> 
> > +static struct btree *mca_bucket_alloc(struct cache_set *c,
> > +				      struct bkey *k, gfp_t gfp)
> > +{
> > +	struct btree *b = kzalloc(sizeof(struct btree), gfp);
> > +	if (!b)
> > +		return NULL;
> > +
> > +	init_rwsem(&b->lock);
> > +	INIT_LIST_HEAD(&b->list);
> > +	INIT_DELAYED_WORK(&b->work, btree_write_work);
> > +	b->c = c;
> > +	closure_init_unlocked(&b->io);
> > +
> > +	mca_data_alloc(b, k, gfp);
> > +	return b->sets[0].data ? b : NULL;
> > +}
> 
> mca_data_alloc() failure path seems like a resource leak but it isn't
> because mca_data_alloc() puts it in free list.  Is the extra level of
> caching necessary?  How is it different from sl?b allocator cache?
> And either way, comments please.

This btree in memory cache code is probably the nastiest, subtlest,
trickiest code in bcache. I have some cleanups in a branch somewhere as
part of some other work that I need to finish off.

The btree_cache_freed list isn't for caching - we never free struct
btrees except on shutdown, to simplify the code. It doesn't cost much
memory since the memory usage is dominated by the buffers struct btree
points to, and those are freed when necessary.

btree_cache_freeable is a small cache for the buffers struct btree
points to (the btree node as it exists on disk + the bset tree). We have
that because high order page allocations can be expensive, and it's
quite common to delete and allocate btree nodes in quick succession -
but it should never grow past 2 or 3 nodes.

I'm adding all this to where the lists are defined, in struct cache_set.

> 
> > +static int mca_reap(struct btree *b, struct closure *cl)
> > +{
> > +	lockdep_assert_held(&b->c->bucket_lock);
> > +
> > +	if (!down_write_trylock(&b->lock))
> > +		return -1;
> > +
> > +	BUG_ON(btree_node_dirty(b) && !b->sets[0].data);
> > +
> > +	if (cl && btree_node_dirty(b))
> > +		btree_write(b, true, NULL);
> > +
> > +	if (cl)
> > +		closure_wait_event_async(&b->io.wait, cl,
> > +			 atomic_read(&b->io.cl.remaining) == -1);
> > +
> > +	if (btree_node_dirty(b) ||
> > +	    atomic_read(&b->io.cl.remaining) != -1 ||
> > +	    work_pending(&b->work.work)) {
> > +		rw_unlock(true, b);
> > +		return -EAGAIN;
> > +	}
> > +
> > +	return 0;
> > +}
> 
> Mixing -1 and -EAGAIN returns usually isn't a good idea.

Yeah. I'm not going to change anything in this code right away, since
it's just to tricky to screw with - but when I finish off those other
patches this code will be improved somewhat (this is already fixed in
that branch).

> > +	b->flags	= 0;
> > +	b->level	= level;
> > +	b->written	= 0;
> > +	b->nsets	= 0;
> > +	for (int i = 0; i < MAX_BSETS; i++)
> > +		b->sets[i].size = 0;
> > +	for (int i = 1; i < MAX_BSETS; i++)
> > +		b->sets[i].data = NULL;
> 
> Why separate loops?

Sceond loop starts at 1, because b->sets[0]->data points to the memory
we have allocated. I suppose that merits a comment.

> 
> > +
> > +	return b;
> > +err:
> > +	if (current->bio_list)
> > +		return ERR_PTR(-EAGAIN);
> 
> What does this test do?

Trying to free up some memory - i.e. reuse some btree nodes - may
require initiating IO to flush the dirty part of the node. If we're
running under generic_make_request(), that IO will never finish and we
would deadlock - returning -EAGAIN causes the cache lookup code to punt
to workqueue and retry.

> 
> Thanks.
> 
> -- 
> tejun
> --
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