linux-kernel - Re: [PATCH 0/4 v3] fs: Remove i

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Message-ID: <CALCETrUgPj=LdLDhdt9AequiMYFSV6B2A6HBVwSsJvyMu1X+6A@mail.gmail.com>
Date:	Tue, 4 Nov 2014 12:24:52 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 0/4 v3] fs: Remove i_devices from struct inode

On Tue, Nov 4, 2014 at 12:16 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Tue, Nov 04, 2014 at 11:52:22AM -0800, Andy Lutomirski wrote:
>
>> Let me try again, though: what if a chardev inode replaced i_cdev with
>> NULL and dropped its kobj reference in iput_final?  This would add a
>> bit over overhead to things that repeatedly open and close the same
>> device node, but I doubt this matters much.
>
> How is it different from what Jan proposed?  The whole problem is that
> iput_final() is too late.  We could clean the reference on close, all
> right, and have it looked up again on every open, but I'm not at all sure
> that situation with opening/closing cdev is never a hot path, especially with
> udev playing silly buggers, etc.

I thought the issue was inodes on the LRU list holding references open.

Anyway, the only expensive part of recreating the reference seems to
be kobj_lookup, and that doesn't look so bad.  If there's a case where
the performance impact would matter at all, maybe that's an argument
for fixing it.

And is udev really opening the same device node more than once?  I can
see once for blkid and such, but I don't see where a second open call
would come from.

--Andy
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