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

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Date:	Thu, 17 Jan 2008 19:20:53 +0300
From:	"Anton Salikhmetov" <salikhmetov@...il.com>
To:	"Miklos Szeredi" <miklos@...redi.hu>
Cc:	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>:
> > > I'm not sure this auto-updating is really needed (POSIX doesn't
> > > mandate it).
> >
> > Peter Shtaubach, author of the first solution for this bug,
> > and Jacob Ostergaard, the reporter of this bug, insist the "auto-update"
> > feature to be implemented.
>
> Can they state their reasons for the insistence?
>
> >  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
>
> Oh, and the four-liner I posted the other day will give you 1) + 2) +
> even more at a small fraction of the complexity.  And tacking on the
> reprotect code will solve the MS_ASYNC issue just the same.
>
> I agree, that having the timestamp updated on sync() is nice, and that
> trivial patch will give you that, and will also update the timestamp
> at least each 30 seconds if the file is being constantly modified,
> even if no explicit syncing is done.
>
> So maybe it's worth a little effort benchmarking how much that patch
> affects the cost of writing to a page.
>
> You could write a little test program like this (if somebody hasn't
> yet done so):
>
>  - do some preparation:
>
>    echo 80 > dirty_ratio
>    echo 80 > dirty_background_ratio
>    echo 30000 > dirty_expire_centisecs
>    sync
>
>  - map a large file, one that fits comfortably into free memory
>  - bring the whole file in, by reading a byte from each page
>  - start the timer
>  - write a byte to each page
>  - stop the timer
>
> It would be most interesting to try this on a filesystem supporting
> nanosecond timestamps.  Anyone know which these are?

The do_wp_page() function is called in mm/memory.c after locking PTE.
And the file_update_time() routine calls the filesystem operation that can
sleep. It's not accepted, I guess.

>
> Miklos
> ----
>
> Index: linux/mm/memory.c
> ===================================================================
> --- linux.orig/mm/memory.c      2008-01-09 21:16:30.000000000 +0100
> +++ linux/mm/memory.c   2008-01-15 21:16:14.000000000 +0100
> @@ -1680,6 +1680,8 @@ gotten:
>  unlock:
>         pte_unmap_unlock(page_table, ptl);
>         if (dirty_page) {
> +               if (vma->vm_file)
> +                       file_update_time(vma->vm_file);
>                 /*
>                  * Yes, Virginia, this is actually required to prevent a race
>                  * with clear_page_dirty_for_io() from clearing the page dirty
> @@ -2313,6 +2315,8 @@ out_unlocked:
>         if (anon)
>                 page_cache_release(vmf.page);
>         else if (dirty_page) {
> +               if (vma->vm_file)
> +                       file_update_time(vma->vm_file);
>                 set_page_dirty_balance(dirty_page, page_mkwrite);
>                 put_page(dirty_page);
>         }
>
>
>
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