From: Anand Jain <anand.jain@oracle.com>
To: Filipe Manana <fdmanana@kernel.org>,
dsterba@suse.cz, linux-btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH] Btrfs: avoid deadlock with memory reclaim due to allocation of devices
Date: Thu, 10 Jan 2019 15:32:07 +0800 [thread overview]
Message-ID: <103742d8-5b13-b080-05b4-070df735df77@oracle.com> (raw)
In-Reply-To: <CAL3q7H5iJ0_g8w2s-2jRMJdZLDT+pqgmtXU5dWQ6A4PjJAUpkg@mail.gmail.com>
On 01/10/2019 03:48 AM, Filipe Manana wrote:
> On Wed, Jan 9, 2019 at 6:27 PM David Sterba <dsterba@suse.cz> wrote:
>>
>> On Thu, Dec 13, 2018 at 09:17:25PM +0000, fdmanana@kernel.org wrote:
>>> - if (list_empty(&fs_devices->resized_devices))
>>> - return;
>>> -
>>> - mutex_lock(&fs_devices->device_list_mutex);
>>> mutex_lock(&fs_info->chunk_mutex);
>>> list_for_each_entry_safe(curr, next, &fs_devices->resized_devices,
>>> resized_list) {
>>> @@ -7309,7 +7306,6 @@ void btrfs_update_commit_device_size(struct btrfs_fs_info *fs_info)
>>> curr->commit_total_bytes = curr->disk_total_bytes;
>>
>> I'm not sure about removing the device_list_mutex that's said to protect
>> the commit_total_bytes (comment in struct btrfs_device).
>>
>> Otherwise the logic is ok, the double lock could happen as you describe.
>>
>> btrfs_update_commit_device_size is called from btrfs_commit_transaction,
>> at the same time as commit_bytes_used. The latter is handled in a
>> similar way in btrfs_update_commit_device_bytes_used, but does not take
>> the device_list_mutex.
>>
>> commit_total_bytes is checked several times (eg. in write_dev_supers) to
>> see if writing the superblock copy is still within the device range.
>>
>> So, without the protected change, it's theoretically possible that a
>> stale value is used for the test and the superblock is either written
>> though it should not, and the other way around.
>>
>> This would require a resize racing at the time of the check. Grow and
>> shrink seem to take chunk_mutex while adjusting all the total/size
>> values, but it's not actually easy to follow as sometimes there are
>> helpers like btrfs_device_set_total_bytes used and sometimes it's direct
>> access.
>>
>> That the device_list_mutex can be safely dropped probably follows from
>> the simple fact that btrfs_update_commit_device_bytes_used is called
>> before write_dev_supers in the same context.
>>
>> But this sounds too simple, given that there are locks taken and
>> released and btrfs_write_and_wait_transaction called between.
>
> Regardless of all that (and honestly I haven't double checked and
> skimmed only through what you said),
> there's a more important aspect I missed before: write_all_supers()
> takes (and needs) the device list mutex,
> therefore this change won't fix the deadlock because of that.
Though this won't fix the problem, this patch is still ok
as its drops the unnecessary device_list_mutex in
btrfs_update_commit_device_size(). So for that if the change log
updated,
Reviewed-by: Anand Jain <anand.jain@oracle.com>
To address the actual problem.
Functions which call btrfs_alloc_device() are..
device_list_add()
close_fs_devices()
btrfs_init_dev_replace_tgtdev()
clone_fs_devices()
btrfs_init_new_device()
Now among the above following holds the device_list_mutex when calling
btrfs_alloc_device()
device_list_add()
close_fs_devices()
clone_fs_devices()
Now among above three, the lock at device_list_add() and
clone_fs_devices() can be ignored, because the reclaim or flush IO can't
take place on these FSID:devices as device_list_add() is called when FS
is not-mounted or in the mounting context, and clone_fs_devices() is
called when the SEED device is still a read-only FS.
And so we have to only worry about close_fs_devices().
close_fs_devices() - I didn't like the way it does, that is allocate a
new struct btrfs_device instead of just zero-ing the struct btrfs_device
during unmount. I guess it was done to avoid RCU warning (not sure) and
if it isn't real issues I am happy to see btrfs_device_alloc() is
dropped in close_fs_devices(). Which means it also fixes the problem
that - you need more memory to unmount an ideal FS.
Thanks.
> thanks
>
>>
>> Referencing this code:
>>
>> 2201 btrfs_update_commit_device_size(fs_info);
>> 2202 btrfs_update_commit_device_bytes_used(cur_trans);
>> 2203
>> 2204 clear_bit(BTRFS_FS_LOG1_ERR, &fs_info->flags);
>> 2205 clear_bit(BTRFS_FS_LOG2_ERR, &fs_info->flags);
>> 2206
>> 2207 btrfs_trans_release_chunk_metadata(trans);
>> 2208
>> 2209 spin_lock(&fs_info->trans_lock);
>> 2210 cur_trans->state = TRANS_STATE_UNBLOCKED;
>> 2211 fs_info->running_transaction = NULL;
>> 2212 spin_unlock(&fs_info->trans_lock);
>> 2213 mutex_unlock(&fs_info->reloc_mutex);
>> 2214
>> 2215 wake_up(&fs_info->transaction_wait);
>> 2216
>> 2217 ret = btrfs_write_and_wait_transaction(trans);
>> 2218 if (ret) {
>> 2219 btrfs_handle_fs_error(fs_info, ret,
>> 2220 "Error while writing out transaction");
>> 2221 mutex_unlock(&fs_info->tree_log_mutex);
>> 2222 goto scrub_continue;
>> 2223 }
>> 2224
>> 2225 ret = write_all_supers(fs_info, 0);
next prev parent reply other threads:[~2019-01-10 7:32 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-13 21:17 [PATCH] Btrfs: avoid deadlock with memory reclaim due to allocation of devices fdmanana
2018-12-14 7:27 ` Nikolay Borisov
2019-01-08 11:51 ` Filipe Manana
2019-01-09 18:26 ` David Sterba
2019-01-09 19:48 ` Filipe Manana
2019-01-10 7:32 ` Anand Jain [this message]
2019-01-10 7:03 ` Nikolay Borisov
2019-01-11 17:17 ` [PATCH v2] " fdmanana
2019-01-14 8:21 ` Anand Jain
2019-01-18 18:07 ` David Sterba
2019-01-25 2:56 ` Anand Jain
2019-01-25 3:40 ` Anand Jain
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