Re: [patch 2/6] fs: no games with DCACHE_UNHASHED

Eric Dumazet
2009-10-15T07:45:23Z

npiggin@suse.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@cosmosbay.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.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@cosmosbay.com>