linux-kernel - Re: PM/hibernate swapfile regression

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Message-Id: <200907252358.26434.rjw@sisk.pl>
Date:	Sat, 25 Jul 2009 23:58:25 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Alan Jenkins <alan-jenkins@...fmail.co.uk>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org,
	"Hans-Joachim Picht" <hans@...ux.vnet.ibm.com>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Arnd Schneider <arnd@...ibm.com>, linux-fsdevel@...r.kernel.org
Subject: Re: PM/hibernate swapfile regression

On Tuesday 21 July 2009, Christoph Hellwig wrote:
> On Tue, Jul 21, 2009 at 11:50:29AM +0100, Alan Jenkins wrote:
> > +struct block_device *bdcopy(struct block_device *bdev)
> > +{
> > +	atomic_inc(&bdev->bd_inode->i_count);
> > +	return bdev;
> > +}
> > +
> > +EXPORT_SYMBOL(bdcopy);
> 
> The function name doesn't make any sense.  You don't copy anything
> here, but you grab a reference to it.  A better name would be bdgrab,
> mirroing the names of functions like igrab.  A kerneldoc comment
> documenting it would also be very helpful.
> 
> Why do you export it?  The swapfile code is not actually modular.

Thanks for the comments.  Does the one below look better?

Rafael

---
From: Alan Jenkins <alan-jenkins@...fmail.co.uk>
Subject: PM / Hibernate: Replace bdget call with simple atomic_inc of i_count

Create bdgrab().  This function copies an existing reference to a
block_device.  It is safe to call from any context.

Hibernation code wishes to copy a reference to the active swap device.
Right now it calls bdget() under a spinlock, but this is wrong because
bdget() can sleep.  It doesn't need a full bdget() because we already
hold a reference to active swap devices (and the spinlock protects
against swapoff).

Signed-off-by: Alan Jenkins <alan-jenkins@...fmail.co.uk>
CC: linux-fsdevel@...r.kernel.org
Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
---
 fs/block_dev.c     |   10 ++++++++++
 include/linux/fs.h |    1 +
 mm/swapfile.c      |    4 ++--
 3 files changed, 13 insertions(+), 2 deletions(-)

Index: linux-2.6/fs/block_dev.c
===================================================================
--- linux-2.6.orig/fs/block_dev.c
+++ linux-2.6/fs/block_dev.c
@@ -564,6 +564,16 @@ struct block_device *bdget(dev_t dev)
 
 EXPORT_SYMBOL(bdget);
 
+/**
+ * bdgrab -- Grab a reference to an already referenced block device
+ * @bdev:	Block device to grab a reference to.
+ */
+struct block_device *bdgrab(struct block_device *bdev)
+{
+	atomic_inc(&bdev->bd_inode->i_count);
+	return bdev;
+}
+
 long nr_blockdev_pages(void)
 {
 	struct block_device *bdev;
Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h
+++ linux-2.6/include/linux/fs.h
@@ -1946,6 +1946,7 @@ extern void putname(const char *name);
 extern int register_blkdev(unsigned int, const char *);
 extern void unregister_blkdev(unsigned int, const char *);
 extern struct block_device *bdget(dev_t);
+extern struct block_device *bdgrab(struct block_device *bdev);
 extern void bd_set_size(struct block_device *, loff_t size);
 extern void bd_forget(struct inode *inode);
 extern void bdput(struct block_device *);
Index: linux-2.6/mm/swapfile.c
===================================================================
--- linux-2.6.orig/mm/swapfile.c
+++ linux-2.6/mm/swapfile.c
@@ -753,7 +753,7 @@ int swap_type_of(dev_t device, sector_t 
 
 		if (!bdev) {
 			if (bdev_p)
-				*bdev_p = bdget(sis->bdev->bd_dev);
+				*bdev_p = bdgrab(sis->bdev);
 
 			spin_unlock(&swap_lock);
 			return i;
@@ -765,7 +765,7 @@ int swap_type_of(dev_t device, sector_t 
 					struct swap_extent, list);
 			if (se->start_block == offset) {
 				if (bdev_p)
-					*bdev_p = bdget(sis->bdev->bd_dev);
+					*bdev_p = bdgrab(sis->bdev);
 
 				spin_unlock(&swap_lock);
 				bdput(bdev);
--
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