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    SubjectRe: [PATCH] vfs: introduce FS_IOC_SYNCFS to sync a single super
    On Mon, 9 Aug 2010 07:33:57 -0700 (PDT)
    Sage Weil <sage@newdream.net> wrote:

    > Currently the only way to sync a single super_block (and not all of them
    > via sync(2)) is via the BLKFLSBUF ioctl on the block device. That also
    > invalidates the bdev mapping, which isn't usually desireable

    Actually you can do

    mount -o remount /dev/whatever

    and it will sync the fs and retain caches.

    > and it
    > doesn't work for non-block file systems.

    And I guess remount will do that also.

    > The ability to sync a single
    > mount can be useful for both applications and administrators (e.g., when
    > other mounts on the system are hung).
    >
    > Introduce a simple ioctl to sync the super associated with an open file.
    > Pass any error returned by sync_filesystem() back to the user.
    >

    The changelog forgot to tell us why this is a useful thing to add.
    What is the use-case?

    > ---
    > fs/ioctl.c | 9 +++++++++
    > include/linux/fs.h | 1 +
    > 2 files changed, 10 insertions(+), 0 deletions(-)
    >
    > diff --git a/fs/ioctl.c b/fs/ioctl.c
    > index 2d140a7..2aabb19 100644
    > --- a/fs/ioctl.c
    > +++ b/fs/ioctl.c
    > @@ -593,6 +593,15 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd,
    > case FS_IOC_FIEMAP:
    > return ioctl_fiemap(filp, arg);
    >
    > + case FS_IOC_SYNCFS:
    > + {
    > + struct super_block *sb = filp->f_dentry->d_sb;
    > + down_read(&sb->s_umount);
    > + error = sync_filesystem(sb);
    > + up_read(&sb->s_umount);
    > + break;
    > + }
    > +

    `mount -o remount' is surely a Linux-specific side-effect and there's
    really no guarantee that Linux will always retain that side-effect.
    OTOH FS_IOC_SYNCFS is linux-specific.

    If we're going to add something like this then it will need to be
    documented in manpages. Supposedly, a cc to linux-api@vger.kernel.org
    will help make all that happen, but I'm not sure who if anyone is
    answering the phone over there?



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