linux-kernel - Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files

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Date:	Fri, 18 Jan 2008 12:22:25 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Anton Salikhmetov <salikhmetov@...il.com>
cc:	Miklos Szeredi <miklos@...redi.hu>, peterz@...radead.org,
	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, akpm@...ux-foundation.org,
	protasnb@...il.com, r.e.wolff@...wizard.nl,
	hidave.darkstar@...il.com, hch@...radead.org
Subject: Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped
 files



On Fri, 18 Jan 2008, Anton Salikhmetov wrote:
> 
> The current solution doesn't hit the performance at all when compared to
> the competitor POSIX-compliant systems. It is faster and does even more
> than the POSIX standard requires.

Your current patches have two problems:
 - they are simply unnecessarily invasive for a relatively simple issue
 - all versions I've looked at closer are buggy too

Example:

	+               if (pte_dirty(*pte) && pte_write(*pte))
	+                       *pte = pte_wrprotect(*pte);

Uhhuh. Looks simple enough. Except it does a non-atomic pte access while 
other CPU's may be accessing it and updating it from their hw page table 
walkers. What will happen? Who knows? I can see lost access bits at a 
minimum.

IOW, this isn't simple code. It's code that it is simple to screw up. In 
this case, you really need to use ptep_set_wrprotect(), for example.

So why not do it in many fewer lines with that simpler vma->dirty flag?

		Linus
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