linux-kernel - Re: Important for fs devs: rcu-walk merged upstream

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Message-ID: <AANLkTi=4mCaZzGXja0Z_VBS+fst8k8_=t1A-eZwfvKiA@mail.gmail.com>
Date:	Fri, 14 Jan 2011 12:21:45 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Important for fs devs: rcu-walk merged upstream

On Fri, Jan 14, 2011 at 3:18 AM, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Thu, 13 Jan 2011, Miklos Szeredi wrote:
>> On Sat, 8 Jan 2011, Nick Piggin wrote:
>> > The vfs-scale branch is now upstream. If you haven't
>> > looked yet, your filesystem is likely to have been
>> > touched, so check it out.
>> >
>> > Also look at Documentation/filesystems/porting and
>> > path-lookup.txt.
>> >
>> > The dcache_lock stuff should have been all done for you
>> > (for in-tree filesystems, I can help out of tree fses with
>> > conversions there if you ping me offline).
>> >
>> > The rcu-walk stuff can be more tricky for your filesystem
>> > to take advantage of.
>> >
>> > If you supply a .d_revalidate, .permission, or .check_acl,
>> > then path walking is going to be slow and unscalable for
>> > you.
>> >
>> > Out of tree filesystems: you _have_ to at least add a line
>> > of code to the above functions in order to specify that
>> > you don't want to participate in rcu-walk.
>> >
>> > Otherwise, you don't have to care about rcu-walk if you
>> > have a legacy or special filesystem like configfs then I'd
>> > advise against anything fancy. But if you have a
>> > userbase and you expect them to actually do any path
>> > lookups into your filesystem, please take a look.
>>
>> One other thing: I know ECHILD is safe since no sane filesystem will
>> return it in its permission or revalidate callbacks, and even if it
>> does that's just a loss of optimization.
>
> And it's not entirely safe either.  A fuse filesystem returning ECHILD
> would make nameidata_dentry_drop_rcu() to BUG.  So some sort of errno
> filter is necessary in the fuse kernel module.

Surely you'd need some filtering anyway? I don't think any function
involving path lookup could sanely return -ECHILD.

That said, it probably is a good idea to have a new errno.
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