linux-kernel - Re: [PATCH 4/5] ext4: fallocate support in ext4

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Message-ID: <20070508105247.GA1950@amitarora.in.ibm.com>
Date:	Tue, 8 May 2007 16:22:47 +0530
From:	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>
To:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
	xfs@....sgi.com, suparna@...ibm.com, cmm@...ibm.com
Subject: Re: [PATCH 4/5] ext4: fallocate support in ext4

On Mon, May 07, 2007 at 10:24:37AM -0500, Dave Kleikamp wrote:
> On Mon, 2007-05-07 at 17:37 +0530, Amit K. Arora wrote:
> > On Thu, May 03, 2007 at 09:31:33PM -0700, Andrew Morton wrote:
> > > On Thu, 26 Apr 2007 23:43:32 +0530 "Amit K. Arora" <aarora@...ux.vnet.ibm.com> wrote:
> 
> > > > +int ext4_fallocate(struct inode *inode, int mode, loff_t offset, loff_t len)
> > > > +{
> > > > +	handle_t *handle;
> > > > +	ext4_fsblk_t block, max_blocks;
> > > > +	int ret, ret2, nblocks = 0, retries = 0;
> > > > +	struct buffer_head map_bh;
> > > > +	unsigned int credits, blkbits = inode->i_blkbits;
> > > > +
> > > > +	/* Currently supporting (pre)allocate mode _only_ */
> > > > +	if (mode != FA_ALLOCATE)
> > > > +		return -EOPNOTSUPP;
> > > > +
> > > > +	if (!(EXT4_I(inode)->i_flags & EXT4_EXTENTS_FL))
> > > > +		return -ENOTTY;
> > > 
> > > So we don't implement fallocate on bitmap-based files!  Well that's huge
> > > news.  The changelog would be an appropriate place to communicate this,
> > > along with reasons why, or a description of the plan to fix it.
> > 
> > Ok. Will add this in the function description as well.
> > 
> > > Also, posix says nothing about fallocate() returning ENOTTY.
> > 
> > Right. I don't seem to find any suitable error from posix description.
> > Can you please suggest an error code which might make more sense here ?
> > Will -ENOTSUPP be ok ? Since we want to say here that we don't support
> > non-extent files.
> 
> Isn't the idea that libc will interpret -ENOTTY, or whatever is returned
> here, and fall back to the current library code to do preallocation?
> This way, the caller of fallocate() will never see this return code, so
> it won't violate posix.

You are right.

But, we still need to "standardize" (and limit) the error codes
which we should return from kernel when we want to fall back on the
library implementation. The posix_fallocate() library function will have
to look for a set of errors from fallocate() system call, upon receiving
which it will do preallocation from user level; or else, it will return
success/error-code returned by the system call to the user.

I think we can make it fall back to library implementation of fallocate,
whenever posix_fallocate() receives any of the following errors from
fallocate() system call:

1. ENOSYS
2. EOPNOTSUPP
3. ENOTTY        (?)

Now the question is - should we limit the set of errors for this purpose
to just 1 & 2 above ? In that case I will need to change the error being
returned here to -EOPNOTSUPP (from current -ENOTTY).

--
Regards,
Amit Arora
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