linux-kernel - Re: [PATCH 7/7][TAKE5] ext4: support new modes

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Date:	Tue, 26 Jun 2007 17:37:52 +0530
From:	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>
To:	Andreas Dilger <adilger@...sterfs.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, David Chinner <dgc@....com>,
	suparna@...ibm.com, cmm@...ibm.com, xfs@....sgi.com
Subject: Re: [PATCH 7/7][TAKE5] ext4: support new modes

On Mon, Jun 25, 2007 at 03:56:25PM -0600, Andreas Dilger wrote:
> On Jun 25, 2007  19:20 +0530, Amit K. Arora wrote:
> > @@ -2499,7 +2500,8 @@ long ext4_fallocate(struct inode *inode,
> >  	 * currently supporting (pre)allocate mode for extent-based
> >  	 * files _only_
> >  	 */
> > -	if (mode != FA_ALLOCATE || !(EXT4_I(inode)->i_flags & EXT4_EXTENTS_FL))
> > +	if (!(EXT4_I(inode)->i_flags & EXT4_EXTENTS_FL) ||
> > +		!(mode == FA_ALLOCATE || mode == FA_RESV_SPACE))
> >  		return -EOPNOTSUPP;
> 
> This should probably just check for the individual flags it can support
> (e.g. no FA_FL_DEALLOC, no FA_FL_DEL_DATA).

Hmm.. I am thinking of a scenario when the file system supports some
individual flags, but does not support a particular combination of them.
Just for example sake, assume we have FA_ZERO_SPACE mode also. Now, if a
file system supports FA_ZERO_SPACE, FA_ALLOCATE, FA_DEALLOCATE and
FA_RESV_SPACE; and no other mode (i.e. FA_UNRESV_SPACE is not supported
for some reason). This means that although we support FA_FL_DEALLOC,
FA_FL_KEEP_SIZE and FA_FL_DEL_DATA flags, but we do not support the
combination of all these flags (which is nothing but FA_UNRESV_SPACE).

> I also thought another proposed flag was to determine whether mtime (and
> maybe ctime) is changed when doing prealloc/dealloc space?  Default should
> probably be to change mtime/ctime, and have FA_FL_NO_MTIME.  Someone else
> should decide if we want to allow changing the file w/o changing ctime, if
> that is required even though the file is not visibly changing.  Maybe the
> ctime update should be implicit if the size or mtime are changing?

Is it really required ? I mean, why should we allow users not to update
ctime/mtime even if the file metadata/data gets updated ? It sounds
a bit "unnatural" to me.
Is there any application scenario in your mind, when you suggest of
giving this flexibility to userspace ?

I think, modifying ctime/mtime should be dependent on the other flags.
E.g., if we do not zero out data blocks on allocation/deallocation,
update only ctime. Otherwise, update ctime and mtime both.

--
Regards,
Amit Arora
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