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

To: "Christoph Hellwig" <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH 1/3] Implement generic freeze feature
From: "Takashi Sato" <t-sato@xxxxxxxxxxxxx>
Date: Thu, 11 Sep 2008 20:11:06 +0900
Cc: "Andrew Morton" <akpm@xxxxxxxxxxxxxxxxxxxx>, "Christoph Hellwig" <hch@xxxxxxxxxxxxx>, "Oleg Nesterov" <oleg@xxxxxxxxxx>, <linux-fsdevel@xxxxxxxxxxxxxxx>, <dm-devel@xxxxxxxxxx>, <viro@xxxxxxxxxxxxxxxxxx>, <linux-ext4@xxxxxxxxxxxxxxx>, <xfs@xxxxxxxxxxx>, <axboe@xxxxxxxxx>, <mtk.manpages@xxxxxxxxxxxxxx>, <linux-kernel@xxxxxxxxxxxxxxx>
In-reply-to: <20080908171020.GA22521@xxxxxxxxxxxxx>
References: <20080908205245t-sato@xxxxxxxxxxxxxxx> <20080908171020.GA22521@xxxxxxxxxxxxx>

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)
+     /* 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++;

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

+     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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