linux-kernel - Re: [PATCH, RFC] check for frozen filesystems in the mmap path

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Message-Id: <20090422132451.62A4.A69D9226@jp.fujitsu.com>
Date:	Wed, 22 Apr 2009 13:49:21 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH, RFC] check for frozen filesystems in the mmap path

> > 2. this logic kill multi thread application.
> > 
> > this logic mean mmap_sem grabbing until unfreeze.
> > it mean othrer thread in the same process can't page-fault although
> > it don't touch frozen-sb.
> > it seems strange.
> 
> Hm, I hadn't thought about this ... On the one hand, ->page_mkwrite can
> already sleep, though a userspace freeze/unfreeze could potentially take
> much much longer.  freeze/unfreeze *should* happen very quickly, but
> nothing enforces that.
> 
> Do you have any suggestions?

One more comment.

I read ioctl_fsfreeze() and freeze_bdev(), it call __fsync_super().
Oh, I don't think __fsync_suepr is very quick.

So, page-fault have one unique characteristics.
if page-fault return 0 without pte change, page-fault is occur again soon.
then, if you need long time waiting, I think you can use following technique.

	unlock mmap_sem
	wait long-time
	lock mmap_sem
	goto out;


it cause page-fault counter increment twice unintesionally.
but no problem. fs-freeze is not freqently event.

Am I missing anything?



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