Re: [PATCH 6/6] bcachefs: rebalance_work

Kent Overstreet
2023-11-02T01:11:32Z

On Wed, Nov 01, 2023 at 10:07:43AM -0700, Nathan Chancellor wrote:
> 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?

Squashed it in, thanks :)