I forgot to say: If you do what I did for NTFS you can also throw
away your custom dentry operations that your patch adds as the dcache
then only holds correctly cased names so you are fine to do case
sensitive dcache lookups at all times. Access via wrongly cased name
will always go to ->lookup inode operation and that is fine because
such lookups almost never happen because majority of users will either
use a GUI in which case all names are always correctly cased as the
names displayed in the GUI are obtained from a ->readdir and thus show
the correct case or they will use the command line in which case they
will be savvy enough to use tab-completion in which case the names are
correct case, too. Tab-completion does not work on wrongly cased
names so you are very unlikely to ever get a wrongly cased name at all.
And yes of course you can on purpose construct a test / benchmark
where having to do the ->lookup each time will be really slow because
you keep creating files and then accessing them by wrongly cased name
on purpose (or whatever) but I would hope that you do not care about
such artificial benchmarks that do not reflect any real-world loads...
Best regards,
Anton
On 23 Oct 2007, at 11:01, Anton Altaparmakov wrote:
Hi,
On 23 Oct 2007, at 08:53, Barry Naujok wrote:
Following is the initial test version of case-insensitive support
for XFS in Linux. It implements case-insensitivity utilising a
Unicode case folding table stored on disk generated from
http://www.unicode.org/Public/UNIDATA/CaseFolding.txt
As the filesystem stores names as Unicode (UTF-8), the "nls"
mount option has been added to support systems not utilising
UTF-8 natively. If the nls mount option is not used, it will
use the default NLS defined in the kernel's config.
To allow case-insensitivity to be a mount option rather than
a mkfs option, the hashes stored on disk are always case-folded.
This is indicated by the new "unicode" bit in the superblock.
This bit also associated with the presence of the case-folding
table on disk.
With the case-folding table on disk, it allows us to upgrade
the table in the future while retaining backwards and forwards
compatibility. It also allows special case tables such as
Turkic case which is supported in this patch set.
The case-insensitive support also installs a couple of
dentry_operations for the XFS inodes: hash and compare.
Currently, there is a couple of outstanding issues with the
dentry cache interaction:
- The first lookup if case-mismatched will continue to
have the mismatched case in the cache. Not really sure
if this is an issue or not. If it is an issue, how
should I resolve it?
- As above, but with a non-existing lookup, then creating
the file with a different case, the first failed lookup
will define the case used. I have partially resolved
this with a memcpy if the two lengths are the same.
How do I fix this if the lengths are different?
(TODO's show the location of this problem.)
Both of the above can be fairly easily fixed if you want. NTFS does
it in the stock kernel.
You would need to change the XFS ->lookup inode operation so that
when it reads the directory to check whether a name exists, if it is
found but the case is not matched, you need to make a copy of the
correctly cased name (if NTFS this is done in fs/ntfs/
dir.c::ntfs_lookup_inode_by_name() if you want to take a look, the
name is stored in the "ntfs_name" structure that is allocated during
the lookup if a case mismatched match is found and this is returned
to the caller).
Then in ->lookup() if you got a correctly cased name structure (if
the name was cased correctly the correctly cased named structure
pointer would be NULL) then you need to replace the dentry passed
into ->lookup with a new one with the correct case. This is a
little complicated because such a dentry may already exist in which
case you have to use the existing one (instantiating it if it was
negative) and if it does not already exist you need to allocate a
new one, instantiate it and then move it over the old one. Again a
little complicated because of disconnected dentries for NFS. But it
is not too bad and it works well in NTFS (see fs/ntfs/
namei.c::ntfs_lookup() the code that does all of this starts at the
"handle_name" goto label).
Doing things this way means that you never have wrong case dentries
in dcache. And this in turn means that things like handling -
>unlink and ->rename inode operations is much easier as the dentry
you receive there is returned from a ->lookup() call thus you know
it is correctly cased already so you can do a case-sensitive match
when looking up the directory entry to remove/rename! (I am afraid
you cannot look at the NTFS code for that as that is not publicly
available yet. )-:)
Best regards,
Anton
Other TODOs:
- support for case-insensitve extended attributes
as a separate mount option.
- Other xfsprogs updates: xfs_repair, xfs_db
--
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/
-
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
