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Date: Thu, 27 Nov 2008 05:10:23 -0500
From: Christoph Hellwig <hch@infradead.org>
To: Eric Dumazet <dada1@cosmosbay.com>
Cc: Christoph Hellwig <hch@infradead.org>, Ingo Molnar <mingo@elte.hu>,
       David Miller <davem@davemloft.net>, "Rafael J. Wysocki" <rjw@sisk.pl>,
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       Christoph Lameter <cl@linux-foundation.org>
Subject: Re: [PATCH 6/6] fs: Introduce kern_mount_special() to mount
	special vfs
Message-ID: <20081127101023.GA1454@infradead.org>
References: <49267694.1030506@cosmosbay.com> <20081121.010508.40225532.davem@davemloft.net> <4926AEDB.10007@cosmosbay.com> <4926D022.5060008@cosmosbay.com> <20081121152148.GA20388@elte.hu> <4926D39D.9050603@cosmosbay.com> <20081121153453.GA23713@elte.hu> <492DDCAB.1070204@cosmosbay.com> <20081127095321.GE13860@infradead.org> <492E70B6.70108@cosmosbay.com>
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On Thu, Nov 27, 2008 at 11:04:38AM +0100, Eric Dumazet wrote:
> Hum.. we have a superblock flag already, but testing it in mntput()/mntget()
> is going to be a litle bit expensive if we add a derefence ?
>
> if (mnt && mnt->mnt_sb->s_flags & MS_SPECIAL) {
>   ...
> }

Well, run a benchmark to see if it makes any difference.  And when it
does please always set the mount flag from the common mount code when
it's set on the superblock, and document that this is the only valid way
to set it.


From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Subject: Re: [PATCH 6/6] fs: Introduce kern_mount_special() to mount
	special vfs
Date: Thu, 27 Nov 2008 05:10:23 -0500
Message-ID: <20081127101023.GA1454@infradead.org>
References: <49267694.1030506@cosmosbay.com> <20081121.010508.40225532.davem@davemloft.net> <4926AEDB.10007@cosmosbay.com> <4926D022.5060008@cosmosbay.com> <20081121152148.GA20388@elte.hu> <4926D39D.9050603@cosmosbay.com> <20081121153453.GA23713@elte.hu> <492DDCAB.1070204@cosmosbay.com> <20081127095321.GE13860@infradead.org> <492E70B6.70108@cosmosbay.com>
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Cc: Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>,
	David Miller <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>,
	"Rafael J. Wysocki" <rjw-KKrjLPT3xs0@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	kernel-testers-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Mike Galbraith <efault-Mmb7MZpHnFY@public.gmane.org>,
	Peter Zijlstra <a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org>,
	Linux Netdev List <netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Christoph Lameter <cl-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
To: Eric Dumazet <dada1-fPLkHRcR87vqlBn2x/YWAg@public.gmane.org>
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On Thu, Nov 27, 2008 at 11:04:38AM +0100, Eric Dumazet wrote:
> Hum.. we have a superblock flag already, but testing it in mntput()/mntget()
> is going to be a litle bit expensive if we add a derefence ?
>
> if (mnt && mnt->mnt_sb->s_flags & MS_SPECIAL) {
>   ...
> }

Well, run a benchmark to see if it makes any difference.  And when it
does please always set the mount flag from the common mount code when
it's set on the superblock, and document that this is the only valid way
to set it.