From: Andy Lutomirski <luto@amacapital.net>
Subject: Re: page fault scalability (ext3, ext4, xfs)
Date: Wed, 14 Aug 2013 23:28:40 -0700
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Cc: Andi Kleen <ak@linux.intel.com>, Theodore Ts'o <tytso@mit.edu>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	LKML <linux-kernel@vger.kernel.org>, xfs@oss.sgi.com,
	Dave Hansen <dave.hansen@intel.com>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>, Jan Kara <jack@suse.cz>,
	"linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>,
	Tim Chen <tim.c.chen@linux.intel.com>
To: David Lang <david@lang.hm>
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On Wed, Aug 14, 2013 at 11:18 PM, David Lang <david@lang.hm> wrote:
> On Wed, 14 Aug 2013, Andy Lutomirski wrote:
>
>>> The big problem with this approach is that not doing the
>>> timestamp update on page faults is going to break the inode change
>>> version counting because for ext4, btrfs and XFS it takes a
>>> transaction to bump that counter. NFS needs to know the moment a
>>> file is changed in memory, not when it is written to disk. Also, NFS
>>> requires the change to the counter to be persistent over server
>>> failures, so it needs to be changed as part of a transaction....
>>
>>
>> NFS can do whatever it wants, although I suspect that even NFS can get
>> away with deferring cmtime updates.
>
>
> NFS already has to do syncs to make sure the data is safe on disk, have a
> flag that NFS can use to make the ctime safe, everyone else can get the
> performance improvement and NFS can have it's slow-but-safe approach.
>

I don't see the current code that updates times for NFS.  I'm not
planning on making any changes that'll affect NFS at all (i.e. I don't
think any flag will be needed), but I'd be more confident if I
understand why it worked in the first place.

(For filesystems that provide page_mkwrite, there hasn't been a
file_update_time call in the core code for several kernel versions.)

--Andy

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