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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Anton Salikhmetov <salikhmetov@gmail.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
	peterz@infradead.org, linux-mm@kvack.org, jakob@unthought.net,
	linux-kernel@vger.kernel.org, valdis.kletnieks@vt.edu,
	riel@redhat.com, ksm@42.dk, staubach@redhat.com,
	jesper.juhl@gmail.com, akpm@linux-foundation.org,
	protasnb@gmail.com, r.e.wolff@bitwizard.nl,
	hidave.darkstar@gmail.com, hch@infradead.org
Subject: Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files
Date: Fri, 18 Jan 2008 12:22:25 -0800 (PST)	[thread overview]
Message-ID: <alpine.LFD.1.00.0801181214440.2957@woody.linux-foundation.org> (raw)
In-Reply-To: <4df4ef0c0801181158s3f783beaqead3d7049d4d3fa7@mail.gmail.com>



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

  reply	other threads:[~2008-01-18 20:23 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-17 22:31 [PATCH -v6 0/2] Fixing the issue with memory-mapped file times Anton Salikhmetov
2008-01-17 22:31 ` [PATCH -v6 1/2] Massive code cleanup of sys_msync() Anton Salikhmetov
2008-01-18  9:33   ` Miklos Szeredi
2008-01-18 10:30     ` Anton Salikhmetov
2008-01-17 22:31 ` [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files Anton Salikhmetov
2008-01-18  9:51   ` Miklos Szeredi
2008-01-18 10:15     ` Peter Zijlstra
2008-01-18 10:25       ` Peter Zijlstra
2008-01-18 10:39         ` Anton Salikhmetov
2008-01-18 17:58         ` Linus Torvalds
2008-01-18 18:11           ` Miklos Szeredi
2008-01-18 18:28             ` Rik van Riel
2008-01-18 18:51               ` Miklos Szeredi
2008-01-18 18:43             ` Linus Torvalds
2008-01-18 18:57               ` Miklos Szeredi
2008-01-18 19:08                 ` Linus Torvalds
2008-01-18 19:22                   ` Miklos Szeredi
2008-01-18 19:35                     ` Linus Torvalds
2008-01-18 19:58                       ` Anton Salikhmetov
2008-01-18 20:22                         ` Linus Torvalds [this message]
2008-01-18 21:03                           ` Anton Salikhmetov
2008-01-18 21:27                             ` Linus Torvalds
2008-01-18 22:04                               ` Anton Salikhmetov
2008-01-18 22:21                                 ` Linus Torvalds
2008-01-18 22:35                                   ` Anton Salikhmetov
2008-01-18 22:32                       ` Ingo Oeser
2008-01-18 22:47                         ` Linus Torvalds
2008-01-18 22:54                           ` Rik van Riel
2008-01-19  0:50                             ` Matt Mackall
2008-01-19  4:25                               ` Rik van Riel
2008-01-19 10:22                               ` Miklos Szeredi
2008-01-19 15:49                                 ` Matt Mackall
2008-01-21 14:25                           ` Peter Staubach
2008-01-21 14:36                             ` Anton Salikhmetov
2008-01-18 10:38       ` Miklos Szeredi
2008-01-18 11:00         ` Peter Zijlstra
2008-01-18 11:17           ` Miklos Szeredi
2008-01-18 11:23             ` Peter Zijlstra
2008-01-18 11:36               ` Miklos Szeredi
2008-01-18  9:40 ` [PATCH -v6 0/2] Fixing the issue with memory-mapped file times Miklos Szeredi
2008-01-18 10:31   ` Anton Salikhmetov
2008-01-18 19:48   ` Anton Salikhmetov
2008-01-19 10:45     ` Miklos Szeredi

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