libfs.c:simple_lookup

libfs.c:simple_lookup

[Jan Engelhardt]

Hello,


simple_lookup() in fs/libfs.c does some extra steps, namely

	dentry->d_op = &simple_dentry_operations;
	d_add(dentry, NULL);

as far as I understand, this creates a negative dentry which will be 
deleted sometime later again. Is not it easier to not create it at all 
(since it is not going to be existent anyway)?


Jan
-- 

Re: libfs.c:simple_lookup

[Dave Kleikamp]

On Mon, 2007-02-19 at 21:09 +0100, Jan Engelhardt wrote:
> Hello,
> 
> 
> simple_lookup() in fs/libfs.c does some extra steps, namely
> 
> 	dentry->d_op = &simple_dentry_operations;
> 	d_add(dentry, NULL);
> 
> as far as I understand, this creates a negative dentry which will be 
> deleted sometime later again. Is not it easier to not create it at all 
> (since it is not going to be existent anyway)?

I'm not sure I understand the question, so allow me to restate a similar
question, and you can tell me if we're asking the same thing.

In general, the negative dentry avoids the overhead of looking up a
non-existent entry more than once by "remembering" that the entry does
not exist.  Since there is almost no overhead to calling
simple_lookup(), would be be better off simply returning without
creating the negative dentry (and letting simple_lookup be called more
often)?

Shaggy
-- 
David Kleikamp
IBM Linux Technology Center

Re: libfs.c:simple_lookup

[Jan Engelhardt]

On Feb 19 2007 15:35, Dave Kleikamp wrote:
>On Mon, 2007-02-19 at 21:09 +0100, Jan Engelhardt wrote:
>> Hello,
>> 
>> 
>> simple_lookup() in fs/libfs.c does some extra steps, namely
>> 
>> 	dentry->d_op = &simple_dentry_operations;
>> 	d_add(dentry, NULL);
>> 
>> as far as I understand, this creates a negative dentry which will be 
>> deleted sometime later again. Is not it easier to not create it at all 
>> (since it is not going to be existent anyway)?
>
>I'm not sure I understand the question, so allow me to restate a similar
>question, and you can tell me if we're asking the same thing.
>
>In general, the negative dentry avoids the overhead of looking up a
>non-existent entry more than once by "remembering" that the entry does
>not exist.

Ah, thanks. That might indeed make sense for disk-based filesystems.

>  Since there is almost no overhead to calling
>simple_lookup(), would be be better off simply returning without
>creating the negative dentry (and letting simple_lookup be called more
>often)?

Yes, that is what I was out at. - The comment above simple_lookup already says
it - if it's not already positive, it does not exist.



Jan
--