Re: [PATCH 6/6] bcachefs: rebalance_work

Nathan Chancellor
2023-11-01T17:03:01Z

Hi Kent,

On Tue, Oct 24, 2023 at 03:14:11PM -0400, Kent Overstreet wrote:
> This adds a new btree, rebalance_work, to eliminate scanning required
> for finding extents that need work done on them in the background - i.e.
> for the background_target and background_compression options.
>
> rebalance_work is a bitset btree, where a KEY_TYPE_set corresponds to an
> extent in the extents or reflink btree at the same pos.
>
> A new extent field is added, bch_extent_rebalance, which indicates that
> this extent has work that needs to be done in the background - and which
> options to use. This allows per-inode options to be propagated to
> indirect extents - at least in some circumstances. In this patch,
> changing IO options on a file will not propagate the new options to
> indirect extents pointed to by that file.
>
> Updating (setting/clearing) the rebalance_work btree is done by the
> extent trigger, which looks at the bch_extent_rebalance field.
>
> Scanning is still requrired after changing IO path options - either just
> for a given inode, or for the whole filesystem. We indicate that
> scanning is required by adding a KEY_TYPE_cookie key to the
> rebalance_work btree: the cookie counter is so that we can detect that
> scanning is still required when an option has been flipped mid-way
> through an existing scan.
>
> Future possible work:
> - Propagate options to indirect extents when being changed
> - Add other IO path options - nr_replicas, ec, to rebalance_work so
> they can be applied in the background when they change
> - Add a counter, for bcachefs fs usage output, showing the pending
> amount of rebalance work: we'll probably want to do this after the
> disk space accounting rewrite (moving it to a new btree)
>
> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

<snip>

> diff --git a/fs/bcachefs/reflink.c b/fs/bcachefs/reflink.c
> index 540c78cd4b0c..dbbdf1955f76 100644
> --- a/fs/bcachefs/reflink.c
> +++ b/fs/bcachefs/reflink.c
> @@ -7,6 +7,7 @@
> #include "inode.h"
> #include "io_misc.h"
> #include "io_write.h"
> +#include "rebalance.h"
> #include "reflink.h"
> #include "subvolume.h"
> #include "super-io.h"
> @@ -252,6 +253,7 @@ s64 bch2_remap_range(struct bch_fs *c,
> struct bpos dst_start = POS(dst_inum.inum, dst_offset);
> struct bpos src_start = POS(src_inum.inum, src_offset);
> struct bpos dst_end = dst_start, src_end = src_start;
> + struct bch_io_opts opts;
> struct bpos src_want;
> u64 dst_done;
> u32 dst_snapshot, src_snapshot;
> @@ -269,6 +271,10 @@ s64 bch2_remap_range(struct bch_fs *c,
> bch2_bkey_buf_init(&new_src);
> trans = bch2_trans_get(c);
>
> + ret = bch2_inum_opts_get(trans, src_inum, &opts);
> + if (ret)
> + goto err;
> +

Not sure if this has been reported or fixed yet but this appears to
introduce a valid clang warning:

fs/bcachefs/reflink.c:275:6: error: variable 'dst_done' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
275 | if (ret)
| ^~~
fs/bcachefs/reflink.c:405:9: note: uninitialized use occurs here
405 | return dst_done ?: ret ?: ret2;
| ^~~~~~~~
fs/bcachefs/reflink.c:275:2: note: remove the 'if' if its condition is always false
275 | if (ret)
| ^~~~~~~~
276 | goto err;
| ~~~~~~~~
fs/bcachefs/reflink.c:258:14: note: initialize the variable 'dst_done' to silence this warning
258 | u64 dst_done;
| ^
| = 0
1 error generated.

I tried to reason my way through a patch but I am a little lost, hence
just the report :)

> bch2_trans_iter_init(trans, &src_iter, BTREE_ID_extents, src_start,
> BTREE_ITER_INTENT);
> bch2_trans_iter_init(trans, &dst_iter, BTREE_ID_extents, dst_start,
> @@ -352,10 +358,13 @@ s64 bch2_remap_range(struct bch_fs *c,
> min(src_k.k->p.offset - src_want.offset,
> dst_end.offset - dst_iter.pos.offset));
>
> - ret = bch2_extent_update(trans, dst_inum, &dst_iter,
> - new_dst.k, &disk_res,
> - new_i_size, i_sectors_delta,
> - true);
> + ret = bch2_bkey_set_needs_rebalance(c, new_dst.k,
> + opts.background_target,
> + opts.background_compression) ?:
> + bch2_extent_update(trans, dst_inum, &dst_iter,
> + new_dst.k, &disk_res,
> + new_i_size, i_sectors_delta,
> + true);
> bch2_disk_reservation_put(c, &disk_res);
> }
> bch2_trans_iter_exit(trans, &dst_iter);
> @@ -386,7 +395,7 @@ s64 bch2_remap_range(struct bch_fs *c,
>
> bch2_trans_iter_exit(trans, &inode_iter);
> } while (bch2_err_matches(ret2, BCH_ERR_transaction_restart));
> -
> +err:
> bch2_trans_put(trans);
> bch2_bkey_buf_exit(&new_src, c);
> bch2_bkey_buf_exit(&new_dst, c);

Cheers,
Nathan