linux-kernel - Re: [PATCH 0/3] ext4: introduce two new ioctls

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Message-ID: <20130624080844.GR29338@dastard>
Date:	Mon, 24 Jun 2013 18:08:44 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Theodore Ts'o <tytso@....edu>, Eric Sandeen <esandeen@...hat.com>,
	Andreas Dilger <adilger@...ger.ca>,
	Namjae Jeon <linkinjeon@...il.com>,
	"adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"a.sangwan@...sung.com" <a.sangwan@...sung.com>,
	Namjae Jeon <namjae.jeon@...sung.com>
Subject: Re: [PATCH 0/3] ext4: introduce two new ioctls

On Sun, Jun 23, 2013 at 11:12:35PM -0400, Theodore Ts'o wrote:
> On Mon, Jun 24, 2013 at 12:44:59PM +1000, Dave Chinner wrote:
> > 
> > Hence, at minimum, this should be a fallocate() operation, not a ext4
> > specific ioctl as it is relatively trivial to implement on most
> > extent based filesystems.
> 
> The fallocate() uses a units of bytes for the offset and length; would
> a FALLOC_FL_COLLAPSE_RANGE be guaranteed to work on any arbitrary
> offset and length?  Or would it only work if the offset and length are
> multiples of the file system blocksize?

There's nothing stopping us from restricting the offset/len to
specific alignments if the operation cannot be done on arbitrary
byte ranges. We do that for direct IO, and the sky hasn't fallen
yet.

> The the EXT4_IOC_TRUNCATE_BLOCK_RANGE interface solves this problem by
> using units of file system blocks (i.e., __u32 start_block), but that
> raises another issue, which is it forces the user space program to
> somehow figure out the file system block size, which seems a bit nasty.

Yeah, exactly. We can do that internally very easily, and EINVAL can
be returned when the alignment is bad just like we do for direct
IO...

But, well, I pine for a generic XFS_IOC_DIOINFO interface so the
filesystem can tell users about alignment restrictions....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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