linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: David Sterba <dsterba@suse.cz>
To: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc: clm@fb.com, josef@toxicpanda.com, dsterba@suse.com,
	anand.jain@oracle.com, linux-btrfs@vger.kernel.org,
	linux-kernel@vger.kernel.org, skhan@linuxfoundation.org,
	gregkh@linuxfoundation.org,
	linux-kernel-mentees@lists.linuxfoundation.org,
	syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
Subject: Re: [PATCH] btrfs: fix rw device counting in __btrfs_free_extra_devids
Date: Wed, 21 Jul 2021 19:59:38 +0200	[thread overview]
Message-ID: <20210721175938.GP19710@twin.jikos.cz> (raw)
In-Reply-To: <20210715103403.176695-1-desmondcheongzx@gmail.com>

On Thu, Jul 15, 2021 at 06:34:03PM +0800, Desmond Cheong Zhi Xi wrote:
> Syzbot reports a warning in close_fs_devices that happens because
> fs_devices->rw_devices is not 0 after calling btrfs_close_one_device
> on each device.
> 
> This happens when a writeable device is removed in
> __btrfs_free_extra_devids, but the rw device count is not decremented
> accordingly. So when close_fs_devices is called, the removed device is
> still counted and we get an off by 1 error.
> 
> Here is one call trace that was observed:
>   btrfs_mount_root():
>     btrfs_scan_one_device():
>       device_list_add();   <---------------- device added
>     btrfs_open_devices():
>       open_fs_devices():
>         btrfs_open_one_device();   <-------- rw device count ++
>     btrfs_fill_super():
>       open_ctree():
>         btrfs_free_extra_devids():
> 	  __btrfs_free_extra_devids();  <--- device removed
> 	  fail_tree_roots:
> 	    btrfs_close_devices():
> 	      close_fs_devices();   <------- rw device count off by 1
> 
> Fixes: cf89af146b7e ("btrfs: dev-replace: fail mount if we don't have replace item with target device")

What this patch did in the last hunk was the rw_devices decrement, but
conditional:

@@ -1080,9 +1071,6 @@ static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices,
                if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
                        list_del_init(&device->dev_alloc_list);
                        clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
-                       if (!test_bit(BTRFS_DEV_STATE_REPLACE_TGT,
-                                     &device->dev_state))
-                               fs_devices->rw_devices--;
                }
                list_del_init(&device->dev_list);
                fs_devices->num_devices--;
---


> @@ -1078,6 +1078,7 @@ static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices,
>  		if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
>  			list_del_init(&device->dev_alloc_list);
>  			clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
> +			fs_devices->rw_devices--;
>  		}
>  		list_del_init(&device->dev_list);
>  		fs_devices->num_devices--;

So should it be reinstated in the original form? The rest of
cf89af146b7e handles unexpected device replace item during mount.

Adding the decrement is correct, but right now I'm not sure about the
corner case when teh devcie has the BTRFS_DEV_STATE_REPLACE_TGT bit set.
The state machine of the device bits and counters is not trivial so
fixing it one way or the other could lead to further syzbot reports if
we don't understand the issue.

  parent reply	other threads:[~2021-07-21 18:02 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-15 10:34 Desmond Cheong Zhi Xi
2021-07-15 11:23 ` Anand Jain
2021-07-15 11:55 ` Nikolay Borisov
2021-07-15 13:11   ` Desmond Cheong Zhi Xi
2021-07-21 13:34     ` David Sterba
2021-07-21 17:59 ` David Sterba [this message]
2021-07-25  6:19   ` Desmond Cheong Zhi Xi
2021-07-26 17:52     ` David Sterba
2021-07-26 23:07       ` Desmond Cheong Zhi Xi
2021-07-25 13:49   ` Anand Jain

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210721175938.GP19710@twin.jikos.cz \
    --to=dsterba@suse.cz \
    --cc=anand.jain@oracle.com \
    --cc=clm@fb.com \
    --cc=desmondcheongzx@gmail.com \
    --cc=dsterba@suse.com \
    --cc=gregkh@linuxfoundation.org \
    --cc=josef@toxicpanda.com \
    --cc=linux-btrfs@vger.kernel.org \
    --cc=linux-kernel-mentees@lists.linuxfoundation.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=skhan@linuxfoundation.org \
    --cc=syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com \
    --subject='Re: [PATCH] btrfs: fix rw device counting in __btrfs_free_extra_devids' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).