linux-kernel - Re: [PATCH -v5 2/2] Updating ctime and mtime at syncing

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Message-Id: <E1JFUrm-0006XG-SB@pomaz-ex.szeredi.hu>
Date:	Thu, 17 Jan 2008 14:33:10 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	salikhmetov@...il.com
CC:	miklos@...redi.hu, linux-mm@...ck.org, jakob@...hought.net,
	linux-kernel@...r.kernel.org, valdis.kletnieks@...edu,
	riel@...hat.com, ksm@...dk, staubach@...hat.com,
	jesper.juhl@...il.com, torvalds@...ux-foundation.org,
	a.p.zijlstra@...llo.nl, akpm@...ux-foundation.org,
	protasnb@...il.com, r.e.wolff@...wizard.nl,
	hidave.darkstar@...il.com, hch@...radead.org
Subject: Re: [PATCH -v5 2/2] Updating ctime and mtime at syncing

> 2008/1/17, Miklos Szeredi <miklos@...redi.hu>:
> > > > 4. Recording the time was the file data changed
> > > >
> > > > Finally, I noticed yet another issue with the previous version of my patch.
> > > > Specifically, the time stamps were set to the current time of the moment
> > > > when syncing but not the write reference was being done. This led to the
> > > > following adverse effect on my development system:
> > > >
> > > > 1) a text file A was updated by process B;
> > > > 2) process B exits without calling any of the *sync() functions;
> > > > 3) vi editor opens the file A;
> > > > 4) file data synced, file times updated;
> > > > 5) vi is confused by "thinking" that the file was changed after 3).
> >
> > Updating the time in remove_vma() would fix this, no?
> 
> We need to save modification time. Otherwise, updating time stamps
> will be confusing the vi editor.

remove_vma() will be called when process B exits, so if the times are
updated there, and the flag is cleared, the times won't be updated
later.

> >
> > > > All these changes to inode.c are unnecessary, I think.
> > >
> > > The first part is necessary to account for "remembering" the modification time.
> > >
> > > The second part is for handling block device files. I cannot see any other
> > > sane way to update file times for them.
> >
> > Use file_update_time(), which will do the right thing.  It will in
> > fact do the same thing as write(2) on the device, which is really what
> > we want.
> >
> > Block devices being mapped for write through different device
> > nodes..., well, I don't think we really need to handle such weird
> > corner cases 100% acurately.
> 
> The file_update_time() cannot be used for implementing
> the "auto-update" feature, because the sync() system call
> doesn't "know" about the file which was memory-mapped.

I'm not sure this auto-updating is really needed (POSIX doesn't
mandate it).

At least split it out into a separate patch, so it can be considered
separately on it's own merit.

I think doing the same with the page-table reprotecting in MS_ASYNC is
also a good idea.  That will leave us with

 1) a base patch: update time just from fsync() and remove_vma()
 2) update time on sync(2) as well
 3) update time on MS_ASYNC as well

I'd happily ack the first one, which would solve the most serious
issues, but have some reservations about the other two.

Miklos
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