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

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Message-ID: <4df4ef0c0801181303o6656832g8b63d2a119a86a9c@mail.gmail.com>
Date:	Sat, 19 Jan 2008 00:03:04 +0300
From:	"Anton Salikhmetov" <salikhmetov@...il.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
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

2008/1/18, Linus Torvalds <torvalds@...ux-foundation.org>:
>
>
> 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.

Before using pte_wrprotect() the vma_wrprotect() routine uses the
pte_offset_map_lock() macro to get the PTE and to acquire the ptl
spinlock. Why did you say that this code was not SMP-safe? It should
be atomic, I think.


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

Neither the dirty flag you suggest, nor the AS_MCTIME flag I've
introduced in my previous solutions solve the following problem:

- mmap()
- a write reference
- msync() with MS_ASYNC
- a write reference
- msync() with MS_ASYNC

The POSIX standard requires the ctime and mtime stamps to be updated
not later than at the second call to msync() with the MS_ASYNC flag.

Some other POSIX-compliant operating system such as HP-UX and FreeBSD
satisfy this POSIX requirement. Linux does not.

>
>                 Linus
>
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