[PATCH] Fix S_DAX setting in __blkdev_get

[PATCH] Fix S_DAX setting in __blkdev_get

[Jon Derrick]

This is a continuation of the HIPRI series [1]. I've dropped the HIPRI
flag patch for now to investigate how to have HIPRI reads and/or writes
in a similar method.

This patch is mainly to try and fix S_DAX setting in __blkdev_get.

[1]: http://lists.infradead.org/pipermail/linux-nvme/2016-May/004606.html

Jon Derrick (1):
  block: Fix S_DAX inode flag locking

 fs/block_dev.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

-- 
1.8.3.1

[PATCH] block: Fix S_DAX inode flag locking

[Jon Derrick]

This patch fixes S_DAX bd_inode i_flag locking to conform to suggested
locking rules. It is presumed that S_DAX is the only valid inode flag
for a block device which subscribes to direct-access, and will restore
any previously set flags if direct-access initialization fails.

This reverts to i_flags behavior prior to
bbab37ddc20bae4709bca8745c128c4f46fe63c5
by allowing other bd_inode flags when DAX is disabled

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
---
 fs/block_dev.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 20a2c02..d41e37f 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1159,6 +1159,20 @@ void bd_set_size(struct block_device *bdev, loff_t size)
 }
 EXPORT_SYMBOL(bd_set_size);
 
+static void bd_add_dax(struct inode *inode)
+{
+	inode_lock(inode);
+	inode->i_flags |= S_DAX;
+	inode_unlock(inode);
+}
+
+static void bd_clear_dax(struct inode *inode)
+{
+	inode_lock(inode);
+	inode->i_flags &= ~S_DAX;
+	inode_unlock(inode);
+}
+
 static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part);
 
 /*
@@ -1172,6 +1186,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 {
 	struct gendisk *disk;
 	struct module *owner;
+	struct inode *inode;
 	int ret;
 	int partno;
 	int perm = 0;
@@ -1198,6 +1213,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 	if (!disk)
 		goto out;
 	owner = disk->fops->owner;
+	inode = bdev->bd_inode;
 
 	disk_block_events(disk);
 	mutex_lock_nested(&bdev->bd_mutex, for_part);
@@ -1206,9 +1222,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 		bdev->bd_queue = disk->queue;
 		bdev->bd_contains = bdev;
 		if (IS_ENABLED(CONFIG_BLK_DEV_DAX) && disk->fops->direct_access)
-			bdev->bd_inode->i_flags = S_DAX;
-		else
-			bdev->bd_inode->i_flags = 0;
+			bd_add_dax(inode);
 
 		if (!partno) {
 			ret = -ENXIO;
@@ -1228,6 +1242,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 					bdev->bd_part = NULL;
 					bdev->bd_disk = NULL;
 					bdev->bd_queue = NULL;
+					bd_clear_dax(inode);
 					mutex_unlock(&bdev->bd_mutex);
 					disk_unblock_events(disk);
 					put_disk(disk);
@@ -1239,7 +1254,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 			if (!ret) {
 				bd_set_size(bdev,(loff_t)get_capacity(disk)<<9);
 				if (!blkdev_dax_capable(bdev))
-					bdev->bd_inode->i_flags &= ~S_DAX;
+					bd_clear_dax(inode);
 			}
 
 			/*
@@ -1276,7 +1291,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 			}
 			bd_set_size(bdev, (loff_t)bdev->bd_part->nr_sects << 9);
 			if (!blkdev_dax_capable(bdev))
-				bdev->bd_inode->i_flags &= ~S_DAX;
+				bd_clear_dax(inode);
 		}
 	} else {
 		if (bdev->bd_contains == bdev) {
@@ -1297,6 +1312,11 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 		put_disk(disk);
 		module_put(owner);
 	}
+	inode_lock(inode);
+	if (inode->i_flags & S_DAX)
+		inode->i_flags = S_DAX;
+	inode_unlock(inode);
+
 	bdev->bd_openers++;
 	if (for_part)
 		bdev->bd_part_count++;
@@ -1309,6 +1329,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
 	bdev->bd_disk = NULL;
 	bdev->bd_part = NULL;
 	bdev->bd_queue = NULL;
+	bd_clear_dax(inode);
 	if (bdev != bdev->bd_contains)
 		__blkdev_put(bdev->bd_contains, mode, 1);
 	bdev->bd_contains = NULL;
-- 
1.8.3.1

Re: [PATCH] block: Fix S_DAX inode flag locking

[Dan Williams]

On Tue, May 17, 2016 at 11:29 AM, Jon Derrick
<jonathan.derrick@intel.com> wrote:
> This patch fixes S_DAX bd_inode i_flag locking to conform to suggested

A "fix" implies that its currently broken.  I don't see how it is, not
until we add an ioctl method or other path that also tries to update
the flags outside of blkdev_get() context.  So, I don't think this
patch stands on its own if you were intending it to be merged
separately.

> locking rules. It is presumed that S_DAX is the only valid inode flag
> for a block device which subscribes to direct-access, and will restore
> any previously set flags if direct-access initialization fails.
>
> This reverts to i_flags behavior prior to
> bbab37ddc20bae4709bca8745c128c4f46fe63c5
> by allowing other bd_inode flags when DAX is disabled
>
> Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
> ---
>  fs/block_dev.c | 31 ++++++++++++++++++++++++++-----
>  1 file changed, 26 insertions(+), 5 deletions(-)
>
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 20a2c02..d41e37f 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1159,6 +1159,20 @@ void bd_set_size(struct block_device *bdev, loff_t size)
>  }
>  EXPORT_SYMBOL(bd_set_size);
>
> +static void bd_add_dax(struct inode *inode)
> +{
> +       inode_lock(inode);
> +       inode->i_flags |= S_DAX;
> +       inode_unlock(inode);
> +}
> +
> +static void bd_clear_dax(struct inode *inode)
> +{
> +       inode_lock(inode);
> +       inode->i_flags &= ~S_DAX;
> +       inode_unlock(inode);
> +}

Since this is inode generic should these helpers be prefixed "i_"
rather than "bd_"?

> +
>  static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part);
>
>  /*
> @@ -1172,6 +1186,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
>  {
>         struct gendisk *disk;
>         struct module *owner;
> +       struct inode *inode;
>         int ret;
>         int partno;
>         int perm = 0;
> @@ -1198,6 +1213,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
>         if (!disk)
>                 goto out;
>         owner = disk->fops->owner;
> +       inode = bdev->bd_inode;
>
>         disk_block_events(disk);
>         mutex_lock_nested(&bdev->bd_mutex, for_part);
> @@ -1206,9 +1222,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
>                 bdev->bd_queue = disk->queue;
>                 bdev->bd_contains = bdev;
>                 if (IS_ENABLED(CONFIG_BLK_DEV_DAX) && disk->fops->direct_access)
> -                       bdev->bd_inode->i_flags = S_DAX;
> -               else
> -                       bdev->bd_inode->i_flags = 0;
> +                       bd_add_dax(inode);
>
>                 if (!partno) {
>                         ret = -ENXIO;
> @@ -1228,6 +1242,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
>                                         bdev->bd_part = NULL;
>                                         bdev->bd_disk = NULL;
>                                         bdev->bd_queue = NULL;
> +                                       bd_clear_dax(inode);
>                                         mutex_unlock(&bdev->bd_mutex);
>                                         disk_unblock_events(disk);
>                                         put_disk(disk);
> @@ -1239,7 +1254,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
>                         if (!ret) {
>                                 bd_set_size(bdev,(loff_t)get_capacity(disk)<<9);
>                                 if (!blkdev_dax_capable(bdev))
> -                                       bdev->bd_inode->i_flags &= ~S_DAX;
> +                                       bd_clear_dax(inode);
>                         }
>
>                         /*
> @@ -1276,7 +1291,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
>                         }
>                         bd_set_size(bdev, (loff_t)bdev->bd_part->nr_sects << 9);
>                         if (!blkdev_dax_capable(bdev))
> -                               bdev->bd_inode->i_flags &= ~S_DAX;
> +                               bd_clear_dax(inode);
>                 }
>         } else {
>                 if (bdev->bd_contains == bdev) {
> @@ -1297,6 +1312,11 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
>                 put_disk(disk);
>                 module_put(owner);
>         }
> +       inode_lock(inode);
> +       if (inode->i_flags & S_DAX)
> +               inode->i_flags = S_DAX;
> +       inode_unlock(inode);
> +

Clear all other flags if S_DAX is set? Why?

>         bdev->bd_openers++;
>         if (for_part)
>                 bdev->bd_part_count++;
> @@ -1309,6 +1329,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
>         bdev->bd_disk = NULL;
>         bdev->bd_part = NULL;
>         bdev->bd_queue = NULL;
> +       bd_clear_dax(inode);

Why?

Re: [PATCH] block: Fix S_DAX inode flag locking

[Jon Derrick]

Hi Dan,

Thanks for the review,


On Tue, May 17, 2016 at 12:34:57PM -0700, Dan Williams wrote:
> On Tue, May 17, 2016 at 11:29 AM, Jon Derrick
> <jonathan.derrick@intel.com> wrote:
> > This patch fixes S_DAX bd_inode i_flag locking to conform to suggested
> 
> A "fix" implies that its currently broken.  I don't see how it is, not
> until we add an ioctl method or other path that also tries to update
> the flags outside of blkdev_get() context.  So, I don't think this
> patch stands on its own if you were intending it to be merged
> separately.
Are there no other paths that can set/clear a bd_inode->i_flags?
If not, that's fine and I'll pack this into the next HIPRI set

> 
> > locking rules. It is presumed that S_DAX is the only valid inode flag
> > for a block device which subscribes to direct-access, and will restore
> > any previously set flags if direct-access initialization fails.
> >
> > This reverts to i_flags behavior prior to
> > bbab37ddc20bae4709bca8745c128c4f46fe63c5
> > by allowing other bd_inode flags when DAX is disabled
> >
> > Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
> > ---
> >  fs/block_dev.c | 31 ++++++++++++++++++++++++++-----
> >  1 file changed, 26 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > index 20a2c02..d41e37f 100644
> > --- a/fs/block_dev.c
> > +++ b/fs/block_dev.c
> > @@ -1159,6 +1159,20 @@ void bd_set_size(struct block_device *bdev, loff_t size)
> >  }
> >  EXPORT_SYMBOL(bd_set_size);
> >
> > +static void bd_add_dax(struct inode *inode)
> > +{
> > +       inode_lock(inode);
> > +       inode->i_flags |= S_DAX;
> > +       inode_unlock(inode);
> > +}
> > +
> > +static void bd_clear_dax(struct inode *inode)
> > +{
> > +       inode_lock(inode);
> > +       inode->i_flags &= ~S_DAX;
> > +       inode_unlock(inode);
> > +}
> 
> Since this is inode generic should these helpers be prefixed "i_"
> rather than "bd_"?
That's fine. bd_acquire/forget also use the inode in its parameter list, but its simple enough to use the bdev instead for these functions or i_ as you suggest

> 
> > +
> >  static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part);
> >
> >  /*
> > @@ -1172,6 +1186,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> >  {
> >         struct gendisk *disk;
> >         struct module *owner;
> > +       struct inode *inode;
> >         int ret;
> >         int partno;
> >         int perm = 0;
> > @@ -1198,6 +1213,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> >         if (!disk)
> >                 goto out;
> >         owner = disk->fops->owner;
> > +       inode = bdev->bd_inode;
> >
> >         disk_block_events(disk);
> >         mutex_lock_nested(&bdev->bd_mutex, for_part);
> > @@ -1206,9 +1222,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> >                 bdev->bd_queue = disk->queue;
> >                 bdev->bd_contains = bdev;
> >                 if (IS_ENABLED(CONFIG_BLK_DEV_DAX) && disk->fops->direct_access)
> > -                       bdev->bd_inode->i_flags = S_DAX;
> > -               else
> > -                       bdev->bd_inode->i_flags = 0;
> > +                       bd_add_dax(inode);
> >
> >                 if (!partno) {
> >                         ret = -ENXIO;
> > @@ -1228,6 +1242,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> >                                         bdev->bd_part = NULL;
> >                                         bdev->bd_disk = NULL;
> >                                         bdev->bd_queue = NULL;
> > +                                       bd_clear_dax(inode);
> >                                         mutex_unlock(&bdev->bd_mutex);
> >                                         disk_unblock_events(disk);
> >                                         put_disk(disk);
> > @@ -1239,7 +1254,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> >                         if (!ret) {
> >                                 bd_set_size(bdev,(loff_t)get_capacity(disk)<<9);
> >                                 if (!blkdev_dax_capable(bdev))
> > -                                       bdev->bd_inode->i_flags &= ~S_DAX;
> > +                                       bd_clear_dax(inode);
> >                         }
> >
> >                         /*
> > @@ -1276,7 +1291,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> >                         }
> >                         bd_set_size(bdev, (loff_t)bdev->bd_part->nr_sects << 9);
> >                         if (!blkdev_dax_capable(bdev))
> > -                               bdev->bd_inode->i_flags &= ~S_DAX;
> > +                               bd_clear_dax(inode);
> >                 }
> >         } else {
> >                 if (bdev->bd_contains == bdev) {
> > @@ -1297,6 +1312,11 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> >                 put_disk(disk);
> >                 module_put(owner);
> >         }
> > +       inode_lock(inode);
> > +       if (inode->i_flags & S_DAX)
> > +               inode->i_flags = S_DAX;
> > +       inode_unlock(inode);
> > +
> 
> Clear all other flags if S_DAX is set? Why?
Code today sets i_flags = S_DAX, clearing all other flags, so I figured that's how it's supposed to be. Though I can't see any reason it has to be that way.


> 
> >         bdev->bd_openers++;
> >         if (for_part)
> >                 bdev->bd_part_count++;
> > @@ -1309,6 +1329,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> >         bdev->bd_disk = NULL;
> >         bdev->bd_part = NULL;
> >         bdev->bd_queue = NULL;
> > +       bd_clear_dax(inode);
> 
> Why?
As far as I can tell, this path is only for cleanup when a whole block device or partition block device fails to get. Your question makes me think I don't understand this part of the code correctly, and admittedly I don't understand it all that well.

All callers of out_clear set an error code on ret, so I assumed all instances of out_clear are errors. There's no reason to keep the S_DAX flag on the bdev this time around. If it resolves itself (for example, GENHD_FL_UP gets set), it can be set again on the next __blkdev_get pass. 

Re: [PATCH] block: Fix S_DAX inode flag locking

[Dave Chinner]

On Tue, May 17, 2016 at 12:34:57PM -0700, Dan Williams wrote:
> On Tue, May 17, 2016 at 11:29 AM, Jon Derrick
> <jonathan.derrick@intel.com> wrote:
> > This patch fixes S_DAX bd_inode i_flag locking to conform to suggested
> 
> A "fix" implies that its currently broken.  I don't see how it is, not
> until we add an ioctl method or other path that also tries to update
> the flags outside of blkdev_get() context.  So, I don't think this
> patch stands on its own if you were intending it to be merged
> separately.
> 
> > locking rules. It is presumed that S_DAX is the only valid inode flag
> > for a block device which subscribes to direct-access, and will restore
> > any previously set flags if direct-access initialization fails.
> >
> > This reverts to i_flags behavior prior to
> > bbab37ddc20bae4709bca8745c128c4f46fe63c5
> > by allowing other bd_inode flags when DAX is disabled
> >
> > Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
> > ---
> >  fs/block_dev.c | 31 ++++++++++++++++++++++++++-----
> >  1 file changed, 26 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > index 20a2c02..d41e37f 100644
> > --- a/fs/block_dev.c
> > +++ b/fs/block_dev.c
> > @@ -1159,6 +1159,20 @@ void bd_set_size(struct block_device *bdev, loff_t size)
> >  }
> >  EXPORT_SYMBOL(bd_set_size);
> >
> > +static void bd_add_dax(struct inode *inode)
> > +{
> > +       inode_lock(inode);
> > +       inode->i_flags |= S_DAX;
> > +       inode_unlock(inode);
> > +}
> > +
> > +static void bd_clear_dax(struct inode *inode)
> > +{
> > +       inode_lock(inode);
> > +       inode->i_flags &= ~S_DAX;
> > +       inode_unlock(inode);
> > +}
> 
> Since this is inode generic should these helpers be prefixed "i_"
> rather than "bd_"?

Probably not, because in general filesystems are responsible for
updating i_flags to reflect on-disk inode configuration and that's
typically done under transaction contexts. e.g.  through ioctl
interfaces to set/clear flags that are stored on disk.  As such,
inode->i_flags is effectively protected by the filesystem specific
locking heirarchy, not the generic inode_lock().

e.g. have a look at XFS storing a persistent "DAX-enabled" flag in
the inode, which can be set/cleared on individual inodes dynamically
by FS_IOC_FSSETXATTR. The XFS i_flags update function assumes
exclusive access to the field as it is called under locked
transaction context. Similar code exists in ext4, btrfs, gfs2,
etc....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

Re: [PATCH] block: Fix S_DAX inode flag locking

[Jan Kara]

On Wed 18-05-16 08:58:42, Dave Chinner wrote:
> On Tue, May 17, 2016 at 12:34:57PM -0700, Dan Williams wrote:
> > On Tue, May 17, 2016 at 11:29 AM, Jon Derrick
> > <jonathan.derrick@intel.com> wrote:
> > > This patch fixes S_DAX bd_inode i_flag locking to conform to suggested
> > 
> > A "fix" implies that its currently broken.  I don't see how it is, not
> > until we add an ioctl method or other path that also tries to update
> > the flags outside of blkdev_get() context.  So, I don't think this
> > patch stands on its own if you were intending it to be merged
> > separately.
> > 
> > > locking rules. It is presumed that S_DAX is the only valid inode flag
> > > for a block device which subscribes to direct-access, and will restore
> > > any previously set flags if direct-access initialization fails.
> > >
> > > This reverts to i_flags behavior prior to
> > > bbab37ddc20bae4709bca8745c128c4f46fe63c5
> > > by allowing other bd_inode flags when DAX is disabled
> > >
> > > Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
> > > ---
> > >  fs/block_dev.c | 31 ++++++++++++++++++++++++++-----
> > >  1 file changed, 26 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > > index 20a2c02..d41e37f 100644
> > > --- a/fs/block_dev.c
> > > +++ b/fs/block_dev.c
> > > @@ -1159,6 +1159,20 @@ void bd_set_size(struct block_device *bdev, loff_t size)
> > >  }
> > >  EXPORT_SYMBOL(bd_set_size);
> > >
> > > +static void bd_add_dax(struct inode *inode)
> > > +{
> > > +       inode_lock(inode);
> > > +       inode->i_flags |= S_DAX;
> > > +       inode_unlock(inode);
> > > +}
> > > +
> > > +static void bd_clear_dax(struct inode *inode)
> > > +{
> > > +       inode_lock(inode);
> > > +       inode->i_flags &= ~S_DAX;
> > > +       inode_unlock(inode);
> > > +}
> > 
> > Since this is inode generic should these helpers be prefixed "i_"
> > rather than "bd_"?
> 
> Probably not, because in general filesystems are responsible for
> updating i_flags to reflect on-disk inode configuration and that's
> typically done under transaction contexts. e.g.  through ioctl
> interfaces to set/clear flags that are stored on disk.  As such,
> inode->i_flags is effectively protected by the filesystem specific
> locking heirarchy, not the generic inode_lock().
>
> e.g. have a look at XFS storing a persistent "DAX-enabled" flag in
> the inode, which can be set/cleared on individual inodes dynamically
> by FS_IOC_FSSETXATTR. The XFS i_flags update function assumes
> exclusive access to the field as it is called under locked
> transaction context. Similar code exists in ext4, btrfs, gfs2,
> etc....

So in case of ext4, we actually do use inode_lock() to protect against
racing IOC_SETFLAGS calls. i_flags is a strange mix and there are a few
(like S_DEAD or S_NOSEC) which are not persistent and those get set /
cleared by VFS. Some of those places (e.g. the clearing in
__vfs_setxattr_noperm()) are not really controlled by the filesystem
AFAICT. So when XFS doesn't use inode_lock() to protect i_flags updates it
can race with VFS on i_flags updates.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] block: Fix S_DAX inode flag locking

[Jan Kara]

On Tue 17-05-16 14:38:05, Jon Derrick wrote:
> On Tue, May 17, 2016 at 12:34:57PM -0700, Dan Williams wrote:
> > On Tue, May 17, 2016 at 11:29 AM, Jon Derrick
> > <jonathan.derrick@intel.com> wrote:
> > > This patch fixes S_DAX bd_inode i_flag locking to conform to suggested
> > 
> > A "fix" implies that its currently broken.  I don't see how it is, not
> > until we add an ioctl method or other path that also tries to update
> > the flags outside of blkdev_get() context.  So, I don't think this
> > patch stands on its own if you were intending it to be merged
> > separately.
> Are there no other paths that can set/clear a bd_inode->i_flags?
> If not, that's fine and I'll pack this into the next HIPRI set

So for block device inodes I don't think anybody else is currently messing
with i_flags but using some lock (and inode_lock() looks fine to me) for
that seems like a reasonable future-proofing. But I think it is enough to
send these patches together with your patch set if it introduces another
user of i_flags in block device inodes. It is always better to see a
concrete reason for the change.

> > > +       inode_lock(inode);
> > > +       if (inode->i_flags & S_DAX)
> > > +               inode->i_flags = S_DAX;
> > > +       inode_unlock(inode);
> > > +
> > 
> > Clear all other flags if S_DAX is set? Why?
>
> Code today sets i_flags = S_DAX, clearing all other flags, so I figured
> that's how it's supposed to be. Though I can't see any reason it has to
> be that way.

IMO that's just over-cautious code and there's no good reason for that.
Inodes are initialized with i_flags == 0. Some i_flags may be set by
previous openers of the block device inode but then these are presumably
consistent. So I'd just remove this zeroing.

> > > @@ -1309,6 +1329,7 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> > >         bdev->bd_disk = NULL;
> > >         bdev->bd_part = NULL;
> > >         bdev->bd_queue = NULL;
> > > +       bd_clear_dax(inode);
> > 
> > Why?
> As far as I can tell, this path is only for cleanup when a whole block
> device or partition block device fails to get. Your question makes me
> think I don't understand this part of the code correctly, and admittedly
> I don't understand it all that well.
> 
> All callers of out_clear set an error code on ret, so I assumed all
> instances of out_clear are errors. There's no reason to keep the S_DAX
> flag on the bdev this time around. If it resolves itself (for example,
> GENHD_FL_UP gets set), it can be set again on the next __blkdev_get pass. 

Yeah, I agree. When we fail to properly set up the block device inode, it
probably makes sense to clear S_DAX just to be sure...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] block: Fix S_DAX inode flag locking

[Dave Chinner]

On Wed, May 18, 2016 at 10:20:39AM +0200, Jan Kara wrote:
> On Wed 18-05-16 08:58:42, Dave Chinner wrote:
> > On Tue, May 17, 2016 at 12:34:57PM -0700, Dan Williams wrote:
> > > On Tue, May 17, 2016 at 11:29 AM, Jon Derrick
> > > <jonathan.derrick@intel.com> wrote:
> > > > This patch fixes S_DAX bd_inode i_flag locking to conform to suggested
> > > 
> > > A "fix" implies that its currently broken.  I don't see how it is, not
> > > until we add an ioctl method or other path that also tries to update
> > > the flags outside of blkdev_get() context.  So, I don't think this
> > > patch stands on its own if you were intending it to be merged
> > > separately.
> > > 
> > > > locking rules. It is presumed that S_DAX is the only valid inode flag
> > > > for a block device which subscribes to direct-access, and will restore
> > > > any previously set flags if direct-access initialization fails.
> > > >
> > > > This reverts to i_flags behavior prior to
> > > > bbab37ddc20bae4709bca8745c128c4f46fe63c5
> > > > by allowing other bd_inode flags when DAX is disabled
> > > >
> > > > Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
> > > > ---
> > > >  fs/block_dev.c | 31 ++++++++++++++++++++++++++-----
> > > >  1 file changed, 26 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > > > index 20a2c02..d41e37f 100644
> > > > --- a/fs/block_dev.c
> > > > +++ b/fs/block_dev.c
> > > > @@ -1159,6 +1159,20 @@ void bd_set_size(struct block_device *bdev, loff_t size)
> > > >  }
> > > >  EXPORT_SYMBOL(bd_set_size);
> > > >
> > > > +static void bd_add_dax(struct inode *inode)
> > > > +{
> > > > +       inode_lock(inode);
> > > > +       inode->i_flags |= S_DAX;
> > > > +       inode_unlock(inode);
> > > > +}
> > > > +
> > > > +static void bd_clear_dax(struct inode *inode)
> > > > +{
> > > > +       inode_lock(inode);
> > > > +       inode->i_flags &= ~S_DAX;
> > > > +       inode_unlock(inode);
> > > > +}
> > > 
> > > Since this is inode generic should these helpers be prefixed "i_"
> > > rather than "bd_"?
> > 
> > Probably not, because in general filesystems are responsible for
> > updating i_flags to reflect on-disk inode configuration and that's
> > typically done under transaction contexts. e.g.  through ioctl
> > interfaces to set/clear flags that are stored on disk.  As such,
> > inode->i_flags is effectively protected by the filesystem specific
> > locking heirarchy, not the generic inode_lock().
> >
> > e.g. have a look at XFS storing a persistent "DAX-enabled" flag in
> > the inode, which can be set/cleared on individual inodes dynamically
> > by FS_IOC_FSSETXATTR. The XFS i_flags update function assumes
> > exclusive access to the field as it is called under locked
> > transaction context. Similar code exists in ext4, btrfs, gfs2,
> > etc....
> 
> So in case of ext4, we actually do use inode_lock() to protect against
> racing IOC_SETFLAGS calls. i_flags is a strange mix and there are a few
> (like S_DEAD or S_NOSEC) which are not persistent and those get set /
> cleared by VFS. Some of those places (e.g. the clearing in
> __vfs_setxattr_noperm()) are not really controlled by the filesystem
> AFAICT. So when XFS doesn't use inode_lock() to protect i_flags updates it
> can race with VFS on i_flags updates.

There are several other filesystems with the same problem. It's not
actually clear what they rules for i_flags are. The comment above
inode_set_flags() is ambiguous at best, and even points this out:

 * In the long run, i_mutex is overkill, and we should probably look
 * at using the i_lock spinlock to protect i_flags, and then make sure
 * it is so documented in include/linux/fs.h and that all code follows
 * the locking convention!!

Perhaps that should be done before the situation gets worse...

Cheers

Dave.
-- 
Dave Chinner
david@fromorbit.com