linux-kernel - Re: [PATCH] f2fs: add ioctl to expose current features

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Date:   Fri, 21 Jul 2017 16:01:36 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     lkml <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [PATCH] f2fs: add ioctl to expose current features

On 07/21, Andreas Dilger wrote:
> > On Jul 21, 2017, at 2:11 PM, Jaegeuk Kim <jaegeuk@...nel.org> wrote:
> > 
> > This patch adds an ioctl to provide feature information to user.
> > For exapmle, SQLite can use this ioctl to detect whether f2fs support atomic
> > write or not.
> 
> Just for reference, ext4 exposes functionality that the kernel supports via
> a file in /sys/fs/ext4/features/{feature_name} so that it is possible to
> check [ -e /sys/fs/ext4/features/lazy_itable_init ] in a shell script also.

Thank you for the info. I'll write a patch to expose sysfs entries as well. :)
For sqlite in Android, I think ioctl would be a simpler interface to check the
feature flag rather than to open another sysfs files, regarding to latency and
additional selinux changes.

> 
> There are also per-filesystem tunables in /sys/fs/ext4/{device}/ and it
> wouldn't be a bad idea to expose per-filesystem features something like
> /sys/fs/{fstype}/{device}/features/{feature_name} so that it is easily
> checked by userspace tools using [ -e $feature ] in bash or access()
> in programs rather than an ioctl.

Yeah, agreed.

Thanks,

> 
> Cheers, Andreas
> 
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> > ---
> > fs/f2fs/f2fs.h |  2 ++
> > fs/f2fs/file.c | 13 +++++++++++++
> > 2 files changed, 15 insertions(+)
> > 
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index e06d4e4b4579..5c46a462b56e 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -112,6 +112,7 @@ struct f2fs_mount_info {
> > 
> > #define F2FS_FEATURE_ENCRYPT	0x0001
> > #define F2FS_FEATURE_BLKZONED	0x0002
> > +#define F2FS_FEATURE_ATOMIC_WRITE 0x0004
> > 
> > #define F2FS_HAS_FEATURE(sb, mask)					\
> > 	((F2FS_SB(sb)->raw_super->feature & cpu_to_le32(mask)) != 0)
> > @@ -308,6 +309,7 @@ static inline bool __has_cursum_space(struct f2fs_journal *journal,
> > 						struct f2fs_flush_device)
> > #define F2FS_IOC_GARBAGE_COLLECT_RANGE	_IOW(F2FS_IOCTL_MAGIC, 11,	\
> > 						struct f2fs_gc_range)
> > +#define	F2FS_IOC_GET_FEATURES		_IOR(F2FS_IOCTL_MAGIC, 12, __u32)
> > 
> > #define F2FS_IOC_SET_ENCRYPTION_POLICY	FS_IOC_SET_ENCRYPTION_POLICY
> > #define F2FS_IOC_GET_ENCRYPTION_POLICY	FS_IOC_GET_ENCRYPTION_POLICY
> > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > index 2706130c261b..0e07b5b18cfa 100644
> > --- a/fs/f2fs/file.c
> > +++ b/fs/f2fs/file.c
> > @@ -2384,6 +2384,16 @@ static int f2fs_ioc_flush_device(struct file *filp, unsigned long arg)
> > 	return ret;
> > }
> > 
> > +static int f2fs_ioc_get_features(struct file *filp, unsigned long arg)
> > +{
> > +	struct inode *inode = file_inode(filp);
> > +	u32 sb_feature = le32_to_cpu(F2FS_I_SB(inode)->raw_super->feature);
> > +
> > +	/* Must validate to set it with SQLite behavior in Android. */
> > +	sb_feature |= F2FS_FEATURE_ATOMIC_WRITE;
> > +
> > +	return put_user(sb_feature, (u32 __user *)arg);
> > +}
> > 
> > long f2fs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> > {
> > @@ -2426,6 +2436,8 @@ long f2fs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> > 		return f2fs_ioc_move_range(filp, arg);
> > 	case F2FS_IOC_FLUSH_DEVICE:
> > 		return f2fs_ioc_flush_device(filp, arg);
> > +	case F2FS_IOC_GET_FEATURES:
> > +		return f2fs_ioc_get_features(filp, arg);
> > 	default:
> > 		return -ENOTTY;
> > 	}
> > @@ -2491,6 +2503,7 @@ long f2fs_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> > 	case F2FS_IOC_DEFRAGMENT:
> > 	case F2FS_IOC_MOVE_RANGE:
> > 	case F2FS_IOC_FLUSH_DEVICE:
> > +	case F2FS_IOC_GET_FEATURES:
> > 		break;
> > 	default:
> > 		return -ENOIOCTLCMD;
> > --
> > 2.13.0.rc1.294.g07d810a77f-goog
> > 
> 
> 
> Cheers, Andreas
> 
> 
> 
> 
>