linux-kernel - Re: vfs-scale, d_revalidate from nfsd

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Message-ID: <AANLkTi=H7Zhe4VMsh_tdrd7Atb9FX8j4AdiMNepXGsj9@mail.gmail.com>
Date:	Fri, 14 Jan 2011 14:12:35 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: vfs-scale, d_revalidate from nfsd

On Fri, Jan 14, 2011 at 2:03 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Fri, Jan 14, 2011 at 01:57:55PM +1100, Nick Piggin wrote:
>> On Fri, Jan 14, 2011 at 1:03 AM, J. R. Okajima <hooanon05@...oo.co.jp> wrote:
>> >
>> > NFSD calls filesystem's ->d_revalidate() with the parameter nd == NULL.
>> > So every
>> > ? ? ? ?if (nd->flags & LOOKUP_RCU)
>> > ? ? ? ? ? ? ? ?return -ECHILD;
>> > code which was added to ->d_revalidate() of FS which supports NFS
>> > exporting will crash.
>> >
>> > If we rewrite it as
>> > ? ? ? ?if (nd && (nd->flags & LOOKUP_RCU))
>> > ? ? ? ? ? ? ? ?return -ECHILD;
>> > the problem may not occur.
>> > But I am not sure whether lookup_one_len() call in NFSD support rcu-walk.
>>
>> Ah, good catch.
>>
>> I'm going to change the d_revalidate API so it takes and inode and rcu-walk
>> flag parameter to make it easier for filesystems to implement rcu-walk.
>>
>> That will take care of this NULL nd case.
>
> Oh, for fuck sake...  Would you at least mind posting your API change
> description to fsdevel before going for it?

Of course. I was discussing it with Miklos yesterday too, but haven't
finished getting a proposal together.

The main idea here would be to just pass in a flags parameter rather
thank poking in nd to get the rcu-walk status. That would solve this
problem and also avoid nd for most filesystems that don't care about
it.


> ->d_revalidate() is one sick puppy and it's intimately involved in atomic
> open rewrite.  Please, *please* don't make that shit even messier...

Right, I did agree with Miklos that using nd for passing rcu-walk
status was going in the wrong direction with that API. Do you
agree?
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