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

[Anton Salikhmetov]

2008/1/18, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>:
>
>
> 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
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/