Re: [RFC PATCH] cleaning up negative dentries when remote directory is changed

[Steve French <smfrench@austin.rr.com>]

Trond Myklebust wrote:
> On Thu, 2005-11-24 at 19:33 -0600, Steve French wrote:
>   
>> Due to the dcache caching negative dentries, files which were deleted on 
>> the client (if it resulted in a negative denty) and soon after readded 
>> on the server would still appear deleted to stat (lookup) at least for 
>> the case of cifs .   Although a readdir would readd the positive dentry, 
>> it would be nice to invalidate negative dentries if the client sees that 
>> the parent directory changed (on the server) so that the negative 
>> dentries would get tossed.   Looking around some other examples, it 
>> looks like at least two (coda, nfs) handle this by calling
>>     shrink_dcache_parent(dentry)
>>     
>
> No they don't. shrink_dcache_parent() gets rid of _all_ unused child
> dentries, which is worse than not caching the negative dentry.
>
> d_drop() is the right thing to do when you are talking about a single
> dentry.
>
>   
>> which is the obvious way to do this (as in the attached patch which 
>> seems to work for cifs and address the reported bug).   Is there a 
>> better way to remove the negative dentries (which may no longer be 
>> negative) under a changed directory?
>>     
>
> The way we do it in NFS is that we check the mtime of the parent
> directory. If the latter has changed since the last time we revalidated
> the negative dentry we call d_drop() in order to force a new lookup (the
> timestamp is stored in dentry->d_time).
>
>   
I don't even see any code in my vfs being called on the negative dentry 
other than
the lookup on the parent e.g:
    ls /mnt/modified_dir/deleted_file

I see that the lookup on modified_dir hits cifs's dentry_revalidate 
routine but there is
no call to dentry_revalidate on "deleted_file" ... does nfs export some 
magic dentry
routines other than revalidate?

> In order to deal with the (rare) case where the server does not support
> mtime, we also have a hard limit on the lifetime of the negative dentry.
>
> Cheers,
>   Trond
>
>
>