Re: [PATCH 2/7] vfs: flush and wait for io when setting the immutable flag via SETFLAGS

[Jan Kara]

On Fri 21-06-19 16:57:07, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> 
> When we're using FS_IOC_SETFLAGS to set the immutable flag on a file, we
> need to ensure that userspace can't continue to write the file after the
> file becomes immutable.  To make that happen, we have to flush all the
> dirty pagecache pages to disk to ensure that we can fail a page fault on
> a mmap'd region, wait for pending directio to complete, and hope the
> caller locked out any new writes by holding the inode lock.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>

Seeing the way this worked out, is there a reason to have separate
vfs_ioc_setflags_flush_data() instead of folding the functionality in
vfs_ioc_setflags_check() (possibly renaming it to
vfs_ioc_setflags_prepare() to indicate it does already some changes)? I
don't see any place that would need these two separated...

> +/*
> + * Flush all pending IO and dirty mappings before setting S_IMMUTABLE on an
> + * inode via FS_IOC_SETFLAGS.  If the flush fails we'll clear the flag before
> + * returning error.
> + *
> + * Note: the caller should be holding i_mutex, or else be sure that
> + * they have exclusive access to the inode structure.
> + */
> +static inline int vfs_ioc_setflags_flush_data(struct inode *inode, int flags)
> +{
> +	int ret;
> +
> +	if (!vfs_ioc_setflags_need_flush(inode, flags))
> +		return 0;
> +
> +	inode_set_flags(inode, S_IMMUTABLE, S_IMMUTABLE);
> +	ret = inode_flush_data(inode);
> +	if (ret)
> +		inode_set_flags(inode, 0, S_IMMUTABLE);
> +	return ret;
> +}

Also this sets S_IMMUTABLE whenever vfs_ioc_setflags_need_flush() returns
true. That is currently the right thing but seems like a landmine waiting
to trip? So I'd just drop the vfs_ioc_setflags_need_flush() abstraction to
make it clear what's going on.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR