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Re: [RFC] add FIEMAP ioctl to efficiently map file allocation

To: David Chinner <dgc@xxxxxxx>
Subject: Re: [RFC] add FIEMAP ioctl to efficiently map file allocation
From: Anton Altaparmakov <aia21@xxxxxxxxx>
Date: Tue, 1 May 2007 19:46:53 +0100
Cc: Nicholas Miell <nmiell@xxxxxxxxxxx>, linux-ext4@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, hch@xxxxxxxxxxxxx
In-reply-to: <20070501142049.GG77450368@melbourne.sgi.com>
References: <20070412110550.GM5967@schatzie.adilger.int> <20070416112252.GJ48531920@melbourne.sgi.com> <20070419002139.GK5967@schatzie.adilger.int> <20070419015426.GM48531920@melbourne.sgi.com> <20070430224401.GX5967@schatzie.adilger.int> <20070501042254.GD77450368@melbourne.sgi.com> <1177994346.3362.5.camel@entropy> <20070501142049.GG77450368@melbourne.sgi.com>
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On 1 May 2007, at 15:20, David Chinner wrote:
On Mon, Apr 30, 2007 at 09:39:06PM -0700, Nicholas Miell wrote:
On Tue, 2007-05-01 at 14:22 +1000, David Chinner wrote:
On Mon, Apr 30, 2007 at 04:44:01PM -0600, Andreas Dilger wrote:
This is actually for future use. Any flags that are added into this
range must be understood by both sides or it should be considered an
error. Flags outside the FIEMAP_FLAG_INCOMPAT do not necessarily need
to be supported. If it turns out that 8 bits is too small a range for
INCOMPAT flags, then we can make 0x01000000 an incompat flag that means
e.g. 0x00ff0000 are also incompat flags also.

Ah, ok. So it's not really a set of "compatibility" flags, it's more a
"compulsory" set. Under those terms, i don't really see why this is
necessary - either the filesystem will understand the flags or it will
return EINVAL or ignore them...

I'm assuming that all flags that will be in the original FIEMAP proposal
will be understood by the implementations. Most filesystems can safely
ignore FLAG_HSM_READ, for example, since they don't support HSM, and for
that matter FLAG_SYNC is probably moot for most filesystems also because
they do block allocation at preprw time.

Exactly my point - so why do we really need to encode a compulsory set of

Because flags have meaning, independent of whether or not the filesystem
understands them. And if the filesystem chooses to ignore critically
important flags (instead of returning EINVAL), bad things may happen.

So, either the filesystem will understand the flag or iff the unknown flag
is in the incompat set, it will return EINVAL or else the unknown flag will
be safely ignored.

My point was that there is a difference between specification and implementation - if the specification says something is compulsory, then they must be implemented in the filesystem. This is easy enough to ensure by code review - we don't need additional interface complexity for this....

You are wrong about this because you are missing the point that you have no code to review. The users that will use those flags are going to be applications that run in user space. Chances are you will never see their code. Heck, they might not even be open source applications... And all applications will run against a multitude of kernels. So version X of the application will run on kernel 2.4.*, 2.6.*, a.b.*, etc... For future expandability of the interface I think it is important to have both compulsory and non-compulsory flags.

For example there is no reason why FIEMAP_HSM_READ needs to be compulsory. Most filesystems do not support HSM so can safely ignore it. And applications that want to read/write the data locations that are obtained with the FIEMAP call will likely always supply FIEMAP_HSM_READ because they want to ensure the file is brought in if it is off line so they definitely want file systems that do not support this flag to ignore it.

And vice versa, an application might specify some weird and funky yet to be developed feature that it expects the FS to perform and if the FS cannot do it (either because it does not support it or because it failed to perform the operation) the application expects the FS to return an error and not to ignore the flag. An example could be the asked for FIEMAP_XATTR_FORK flag. If that is implemented, and the FS ignores it it will return the extent map for the file data instead of the XATTR_FORK! Not what the application wanted at all. Ouch! So this is definitely a compulsory flag if I ever saw one.

So as you see you must support both voluntary and compulsory flags...

Also consider what I said above about different kernels. A new feature is implemented in kernel 2.8.13 say that was not there before and an application is updated to use that feature. There will be lots of instances where that application will still be run on older kernels where this feature does not exist. Depending on the feature it may be quite sensible to simply ignore in the kernel that the application set an unknown flag whilst for a different feature it may be the opposite.

Best regards,

Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer, http://www.linux-ntfs.org/

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