Re: [PATCH 2/6] btrfs: add cancelable chunk relocation support

[David Sterba <dsterba@suse.cz>]

On Fri, May 21, 2021 at 09:21:29AM -0400, Josef Bacik wrote:
> On 5/21/21 8:06 AM, David Sterba wrote:
> > Add support code that will allow canceling relocation on the chunk
> > granularity. This is different and independent of balance, that also
> > uses relocation but is a higher level operation and manages it's own
> > state and pause/cancelation requests.
> > 
> > Relocation is used for resize (shrink) and device deletion so this will
> > be a common point to implement cancelation for both. The context is
> > entirely in btrfs_relocate_block_group and btrfs_recover_relocation,
> > enclosing one chunk relocation. The status bit is set and unset between
> > the chunks. As relocation can take long, the effects may not be
> > immediate and the request and actual action can slightly race.
> > 
> > The fs_info::reloc_cancel_req is only supposed to be increased and does
> > not pair with decrement like fs_info::balance_cancel_req.
> > 
> > Signed-off-by: David Sterba <dsterba@suse.com>
> > ---
> >   fs/btrfs/ctree.h      |  9 +++++++
> >   fs/btrfs/disk-io.c    |  1 +
> >   fs/btrfs/relocation.c | 60 ++++++++++++++++++++++++++++++++++++++++++-
> >   3 files changed, 69 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> > index a142e56b6b9a..3dfc32a3ebab 100644
> > --- a/fs/btrfs/ctree.h
> > +++ b/fs/btrfs/ctree.h
> > @@ -565,6 +565,12 @@ enum {
> >   	 */
> >   	BTRFS_FS_BALANCE_RUNNING,
> >   
> > +	/*
> > +	 * Indicate that relocation of a chunk has started, it's set per chunk
> > +	 * and is toggled between chunks.
> > +	 */
> > +	BTRFS_FS_RELOC_RUNNING,
> > +
> >   	/* Indicate that the cleaner thread is awake and doing something. */
> >   	BTRFS_FS_CLEANER_RUNNING,
> >   
> > @@ -871,6 +877,9 @@ struct btrfs_fs_info {
> >   	struct btrfs_balance_control *balance_ctl;
> >   	wait_queue_head_t balance_wait_q;
> >   
> > +	/* Cancelation requests for chunk relocation */
> > +	atomic_t reloc_cancel_req;
> > +
> >   	u32 data_chunk_allocations;
> >   	u32 metadata_ratio;
> >   
> > diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> > index 8c3db9076988..93c994b78d61 100644
> > --- a/fs/btrfs/disk-io.c
> > +++ b/fs/btrfs/disk-io.c
> > @@ -2251,6 +2251,7 @@ static void btrfs_init_balance(struct btrfs_fs_info *fs_info)
> >   	atomic_set(&fs_info->balance_cancel_req, 0);
> >   	fs_info->balance_ctl = NULL;
> >   	init_waitqueue_head(&fs_info->balance_wait_q);
> > +	atomic_set(&fs_info->reloc_cancel_req, 0);
> >   }
> >   
> >   static void btrfs_init_btree_inode(struct btrfs_fs_info *fs_info)
> > diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> > index b70be2ac2e9e..9b84eb86e426 100644
> > --- a/fs/btrfs/relocation.c
> > +++ b/fs/btrfs/relocation.c
> > @@ -2876,11 +2876,12 @@ int setup_extent_mapping(struct inode *inode, u64 start, u64 end,
> >   }
> >   
> >   /*
> > - * Allow error injection to test balance cancellation
> > + * Allow error injection to test balance/relocation cancellation
> >    */
> >   noinline int btrfs_should_cancel_balance(struct btrfs_fs_info *fs_info)
> >   {
> >   	return atomic_read(&fs_info->balance_cancel_req) ||
> > +		atomic_read(&fs_info->reloc_cancel_req) ||
> >   		fatal_signal_pending(current);
> >   }
> >   ALLOW_ERROR_INJECTION(btrfs_should_cancel_balance, TRUE);
> > @@ -3780,6 +3781,47 @@ struct inode *create_reloc_inode(struct btrfs_fs_info *fs_info,
> >   	return inode;
> >   }
> >   
> > +/*
> > + * Mark start of chunk relocation that is cancelable. Check if the cancelation
> > + * has been requested meanwhile and don't start in that case.
> > + *
> > + * Return:
> > + *   0             success
> > + *   -EINPROGRESS  operation is already in progress, that's probably a bug
> > + *   -ECANCELED    cancelation request was set before the operation started
> > + */
> > +static int reloc_chunk_start(struct btrfs_fs_info *fs_info)
> > +{
> > +	if (test_and_set_bit(BTRFS_FS_RELOC_RUNNING, &fs_info->flags)) {
> > +		/* This should not happen */
> > +		btrfs_err(fs_info, "reloc already running, cannot start");
> > +		return -EINPROGRESS;
> > +	}
> > +
> > +	if (atomic_read(&fs_info->reloc_cancel_req) > 0) {
> > +		btrfs_info(fs_info, "chunk relocation canceled on start");
> > +		/*
> > +		 * On cancel, clear all requests but let the caller mark
> > +		 * the end after cleanup operations.
> > +		 */
> > +		atomic_set(&fs_info->reloc_cancel_req, 0);
> > +		return -ECANCELED;
> > +	}
> > +	return 0;
> > +}
> > +
> > +/*
> > + * Mark end of chunk relocation that is cancelable and wake any waiters.
> > + */
> > +static void reloc_chunk_end(struct btrfs_fs_info *fs_info)
> > +{
> > +	/* Requested after start, clear bit first so any waiters can continue */
> > +	if (atomic_read(&fs_info->reloc_cancel_req) > 0)
> > +		btrfs_info(fs_info, "chunk relocation canceled during operation");
> > +	clear_and_wake_up_bit(BTRFS_FS_RELOC_RUNNING, &fs_info->flags);
> > +	atomic_set(&fs_info->reloc_cancel_req, 0);
> > +}
> > +
> >   static struct reloc_control *alloc_reloc_control(struct btrfs_fs_info *fs_info)
> >   {
> >   	struct reloc_control *rc;
> > @@ -3862,6 +3904,12 @@ int btrfs_relocate_block_group(struct btrfs_fs_info *fs_info, u64 group_start)
> >   		return -ENOMEM;
> >   	}
> >   
> > +	ret = reloc_chunk_start(fs_info);
> > +	if (ret < 0) {
> > +		err = ret;
> > +		goto out_end;
> > +	}
> 
> This needs a btrfs_put_block_group(bg) so we don't leak the bg.  Thanks,

Indeed, thanks, As there are no other remaining things to fix I won't
repost the whole series just for this. I've fixed it in misc-next, where
the exit block looks like

@@ -3952,7 +4000,9 @@ int btrfs_relocate_block_group(struct btrfs_fs_info *fs_info, u64 group_start)
        if (err && rw)
                btrfs_dec_block_group_ro(rc->block_group);
        iput(rc->data_inode);
+out_put_bg:
        btrfs_put_block_group(rc->block_group);
+       reloc_chunk_end(fs_info);
        free_reloc_control(rc);
        return err;
 }

and the failure jumps to out_put_bg, ie. restoring the ro count.