linux-kernel - Re: [PATCH] btrfs: let btrfs_delete_unused

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Message-ID: <1473049955.14093.37.camel@hgst.com>
Date:   Mon, 5 Sep 2016 04:32:37 +0000
From:   Naohiro Aota <Naohiro.Aota@...t.com>
To:     "jbacik@...com" <jbacik@...com>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dsterba@...e.com" <dsterba@...e.com>, "clm@...com" <clm@...com>
Subject: Re: [PATCH] btrfs: let btrfs_delete_unused_bgs() to clean relocated
 bgs

2016-09-02 (金) の 09:35 -0400 に Josef Bacik さんは書きました:
> On 09/02/2016 03:46 AM, Naohiro Aota wrote:
> > 
> > Currently, btrfs_relocate_chunk() is removing relocated BG by
> > itself. But
> > the work can be done by btrfs_delete_unused_bgs() (and it's better
> > since it
> > trim the BG). Let's dedupe the code.
> > 
> > While btrfs_delete_unused_bgs() is already hitting the relocated
> > BG, it
> > skip the BG since the BG has "ro" flag set (to keep balancing BG
> > intact).
> > On the other hand, btrfs cannot drop "ro" flag here to prevent
> > additional
> > writes. So this patch make use of "removed" flag.
> > btrfs_delete_unused_bgs() now detect the flag to distinguish
> > whether a
> > read-only BG is relocating or not.
> > 
> 
> This seems racey to me.  We remove the last part of the block group,
> it ends up 
> on the unused_bgs_list, we process this list, see that removed isn't
> set and we 
> skip it, then later we set removed, but it's too late.  I think the
> right way is 
> to actually do a transaction, set ->removed, manually add it to the 
> unused_bgs_list if it's not already, then end the transaction.  This
> way we are 
> guaranteed to have the bg on the list when it is ready to be
> removed.  This is 
> my analysis after looking at it for 10 seconds after being awake for
> like 30 
> minutes so if I'm missing something let me know.  Thanks,

I don't think a race will happen. Since we are holding
delete_unused_bgs_mutex here, btrfs_delte_unused_bgs() checks ->removed
flag after we unlock the mutex i.e. we setup the flag properly. For a
case btrfs_delete_usused_bgs() checks the BG before we hold
delte_unused_bgs_mutex, then that BG is removed by it (if it's empty)
and btrfs_relocate_chunk() should never see it.

Regards,
Naohiro