linux-ext4 - Re: [PATCH 1/3] Implement generic freeze feature

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Message-ID: <059C75EDB49641A49EF986D81EED4200@nsl.ad.nec.co.jp>
Date:	Thu, 11 Sep 2008 20:11:06 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"Christoph Hellwig" <hch@...radead.org>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Christoph Hellwig" <hch@...radead.org>,
	"Oleg Nesterov" <oleg@...sign.ru>, <linux-fsdevel@...r.kernel.org>,
	<dm-devel@...hat.com>, <viro@...IV.linux.org.uk>,
	<linux-ext4@...r.kernel.org>, <xfs@....sgi.com>, <axboe@...nel.dk>,
	<mtk.manpages@...glemail.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Implement generic freeze feature

Hi,

Christoph Hellwig wrote:
>> --- linux-2.6.27-rc5.org/fs/block_dev.c 2008-08-29 07:52:02.000000000 +0900
>> +++ linux-2.6.27-rc5-freeze/fs/block_dev.c 2008-09-05 20:00:29.000000000 +0900
>> @@ -285,6 +285,8 @@ static void init_once(void *foo)
>>      INIT_LIST_HEAD(&bdev->bd_holder_list);
>>  #endif
>>      inode_init_once(&ei->vfs_inode);
>> +     /* Initialize mutex for freeze. */
>> +     mutex_init(&bdev->bd_fsfreeze_mutex);
>
> Why not just freeze_mutex?

The Linux kernel has already had the name of "freezer" in the part of
power-management.  So I named the above mutex "fsfreeze"
(filesystem freeze) to distinguish it from the existent "freezer".
Andrew pointed it out.

>>  struct super_block *freeze_bdev(struct block_device *bdev)
>>  {
>>       struct super_block *sb;
>>
>> +    mutex_lock(&bdev->bd_fsfreeze_mutex);
>> +    if (bdev->bd_fsfreeze_count > 0) {
>> +        bdev->bd_fsfreeze_count++;
>> +        sb = get_super(bdev);
>> +        mutex_unlock(&bdev->bd_fsfreeze_mutex);
>> +        return sb;
>> +    }
>> +    bdev->bd_fsfreeze_count++;
>> +
>>      down(&bdev->bd_mount_sem);
>
> Note that we still have duplication with the bd_mount_sem.  I think
> you should look into getting rid of it and instead do a check of
> the freeze_count under proper freeze_mutex protection.

In the original implementation,
while the filesystem is frozen, subsequent mounts wait for unfreeze
with the semaphore (bd_mount_sem).
But if we replace the semphore with the check of the freeze_count,
subsequent mounts will abort.
I think the original behavior shouldn't be changed, so the existing bd_mount_sem
is better.

>> @@ -244,6 +274,8 @@ void thaw_bdev(struct block_device *bdev
>>  }
>>
>>        up(&bdev->bd_mount_sem);
>> +     mutex_unlock(&bdev->bd_fsfreeze_mutex);
>> +     return 0;
>
> Why do you add a return value here if we always return 0 anyway?

I forgot to remove the unneeded return value in above old patch.
But I need to implement a return value in the new patch
because thaw_bdev() needs to return an IO error which occurs
in unlockfs().
Eric pointed it out.

Cheers, Takashi

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