linux-kernel - Re: [patch 2/6] fs: no games with DCACHE

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Message-ID: <20091015081343.GC4262@wotan.suse.de>
Date:	Thu, 15 Oct 2009 10:13:43 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org,
	Ian Kent <raven@...maw.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Miklos Szeredi <mszeredi@...e.cz>
Subject: Re: [patch 2/6] fs: no games with DCACHE_UNHASHED

On Thu, Oct 15, 2009 at 09:44:35AM +0200, Eric Dumazet wrote:
> npiggin@...e.de a écrit :
> > (this is in -mm)
> > 
> > Filesystems outside the regular namespace do not have to clear DCACHE_UNHASHED
> > in order to have a working /proc/$pid/fd/XXX. Nothing in proc prevents the
> > fd link from being used if its dentry is not in the hash.
> > 
> > Also, it does not get put into the dcache hash if DCACHE_UNHASHED is clear;
> > that depends on the filesystem calling d_add or d_rehash.
> > 
> > So delete the misleading comments and needless code.
> > 
> 
> This was added in commit 304e61e6fbadec586dfe002b535f169a04248e49
> 
>     [PATCH] net: don't insert socket dentries into dentry_hashtable
>     
>     We currently insert socket dentries into the global dentry hashtable.  This
>     is suboptimal because there is currently no way these entries can be used
>     for a lookup().  (/proc/xxx/fd/xxx uses a different mechanism).  Inserting
>     them in dentry hashtable slows dcache lookups.
>     
>     To let __dpath() still work correctly (ie not adding a " (deleted)") after
>     dentry name, we do :
>     
>     - Right after d_alloc(), pretend they are hashed by clearing the
>       DCACHE_UNHASHED bit.
>     
>     - Call d_instantiate() instead of d_add() : dentry is not inserted in
>       hash table.
>     
>       __dpath() & friends work as intended during dentry lifetime.
>     
>     - At dismantle time, once dput() must clear the dentry, setting again
>       DCACHE_UNHASHED bit inside the custom d_delete() function provided by
>       socket code, so that dput() can just kill_it.
>     
>     Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
>     Cc: Al Viro <viro@...iv.linux.org.uk>
>     Acked-by: "David S. Miller" <davem@...emloft.net>
>     Signed-off-by: Andrew Morton <akpm@...l.org>
>     Signed-off-by: Linus Torvalds <torvalds@...l.org>
> 
> 
> 
> Back in 2006, we had to perform this hack in order to not leak '(deleted)' in __d_path()
> 
> if (!IS_ROOT(dentry) && d_unhashed(dentry) &&
> 	(prepend(&end, &buflen, " (deleted)", 10) != 0)) 
> 		goto Elong;
> 
> In current kernel this part became :
> 
> if (d_unlinked(dentry) &&
> 	(prepend(&end, &buflen, " (deleted)", 10) != 0))
> 		goto Elong;
> 
> 
> So your cleanup seems good, thanks !
> 
> Acked-by: Eric Dumazet <dada1@...mosbay.com>

Ahh, hmm d_unlinked() is exactly the same code. OK, so I think this shows
why I'm an idiot. I didn't think about the __d_path output. Though the
comments are still misleading, in my defence (and my test programs did not
use any anoninode fds)...

Now both sockets and pipes define a d_dname so they are OK, but anon_inodes
does not. I think they should probably be made to just provide a d_dname
anyway so we can have the familiar format of "pseudofs:[ino]" rather than
"[pseudofs]" that we have now.

That should make this patch work for anon_inodes.c as well.

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