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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@infradead.org>
References: <20080908205245t-sato@mail.jp.nec.com> <20080908171020.GA22521@infradead.org>
Sender: xfs-bounce@xxxxxxxxxxx
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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