Re: d_splice_alias() problem.

[Nikita Danilov <Nikita@Namesys.COM>]

Neil Brown writes:
 > On Friday April 23, Nikita@Namesys.COM wrote:
 > > Hello,
 > > 
 > > for some time I am observing that during stress tests over NFS
 > > 
 > >    shrink_slab->...->prune_dcache()->prune_one_dentry()->...->iput()
 > > 
 > > is called on inode with ->i_nlink == 0 which results in truncate and
 > > file deletion. This is wrong in general (file system is re-entered), and
 > > deadlock prone on some file systems.
 > > 
 > > After some debugging, I tracked problem down the to d_splice_alias()
 > > failing to identify dentries when necessary.
 > > 
 > > Suppose we have an inode with ->i_nlink == 1. It's accessed over NFS and
 > > DCACHE_DISCONNECTED dentry D1 is created for it. Then, unlink request
 > > comes for this file. nfsd looks name up in the parent directory
 > > (nfsd_unlink()->lookup_one_len()). File system back-end uses
 > > d_splice_alias(), but it only works for directories and we end up with
 > > second (this time connected) dentry D2.
 > > 
 > > D2 is successfully unlinked, file has ->i_nlink == 0, and ->i_count == 1
 > > from D1, and when prune_dcache() hits D1 bad things happen.
 > > 
 > > It's hard to imagine how new name can be identified with one among
 > > multiple anonymous dentries, which is necessary for
 > > NFSEXP_NOSUBTREECHECK export to work reliably.
 > > 
 > > One possible work-around is to forcibly destroy all remaining
 > > DCACHE_DISCONNECTED dentries when ->i_nlink drops to zero, but I am not
 > > sure that this is possible and solves all problems of having more
 > > dentries than there are nlinks.
 > > 
 > > Nikita.
 > 
 > If I understand you correctly, the main problem is that a disconnected
 > dentry can hold an inode active after the last link has been removed.
 > The file will not then be truncated and removed until memory pressure
 > flushes the disconnected dentry from the dcache.
 > 
 > This problem can be resolved by making sure that an inode never has
 > both a connected and a disconnected dentry.
 > 
 > This is already the case for directories (as they must only have one
 > dentry), but it is not the case for non-directories.
 > 
 > The following patch tries to address this.  It is a "technology
 > preview" in that the only testing I have done is that it compiles OK. 

I have a test where such situation is reproducible and will give patch a
try.

Also, Al Viro pointed to me that it's not clear why DCACHE_DISCONNECTED
dentry is DCACHE_HASHED at all. If it were unhashed, last dput (done by
nfsd thread) would destroy it, truncating file if necessary.

 > 

Nikita.