Re: [PATCH 6/6] bcachefs: rebalance_work

Nathan Chancellor
2023-11-01T17:07:49Z

On Wed, Nov 01, 2023 at 10:02:55AM -0700, Nathan Chancellor wrote:
> 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 :)

Actually, it just seems like dst_done should be explicitly zero
initialized, so that ret is used as the return value. Don't know why I
was as confused as I was :) would you like a formal patch or to just
squash it in?

diff --git a/fs/bcachefs/reflink.c b/fs/bcachefs/reflink.c
index eb31df605c2e..6e1bfe9feb59 100644
--- a/fs/bcachefs/reflink.c
+++ b/fs/bcachefs/reflink.c
@@ -255,7 +255,7 @@ s64 bch2_remap_range(struct bch_fs *c,
struct bpos dst_end = dst_start, src_end = src_start;
struct bch_io_opts opts;
struct bpos src_want;
- u64 dst_done;
+ u64 dst_done = 0;
u32 dst_snapshot, src_snapshot;
int ret = 0, ret2 = 0;

> > 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
>