Re: [RFC PATCH 0/4] Support vranges on files

[Minchan Kim <minchan@kernel.org>]

On Tue, Apr 09, 2013 at 03:36:20PM -0700, John Stultz wrote:
> On 04/08/2013 10:07 PM, Minchan Kim wrote:
> >On Mon, Apr 08, 2013 at 08:27:50PM -0700, John Stultz wrote:
> >>marked volatile, it should remain volatile until someone who has the
> >>file open marks it as non-volatile.  The only time we clear the
> >>volatility is when the file is closed by all users.
> >Yes. We need it that clear volatile ranges when the file is closed
> >by ball users. That's what we need and blow my concern out.
> 
> Ok, sorry this wasn't more clear. In all the implementations I've
> pushed, the volatility only persists as long as someone holds the
> file open. Once its closed by all users, the volatility is cleared.

I now confirmed it with your implementation.
Sorry for the confusing without looking into your code in detail. :(

> 
> Hopefully that calms your worries here. :)

Yeb.

> 
> 
> 
> >>I think the concern about surprising an application that isn't
> >>expecting volatility is odd, since if an application jumped in and
> >>punched a hole in the data, that could surprise other applications
> >>as well.  If you're going to use a file that can be shared,
> >>applications have to deal with potential changes to that file by
> >>others.
> >True. My concern is delayed punching without any client of fd and
> >there is no interface to detect some range of file is volatile state or
> >not. It means anyone mapped a file with shared could encunter SIGBUS
> >although he try to best effort to check it with lsof before using.
> 
> I'll grant the SIGBUG semantics create the potential for stranger
> behavior then usual, but I think the use cases are still attractive
> enough to try to make it work.

Indeed.

> 
> 
> >>To me, the value in using volatile ranges on the file data is
> >>exactly because the file data can be shared. So it makes sense to me
> >>to have the volatility state be like the data in the file. I guess
> >>the only exception in my case is that if all the references to a
> >>file are closed, we can clear the volatility (since we don't have a
> >>sane way for the volatility to persist past that point).
> >Agree if you provide to clear out volatility when file are closed by
> >all stakeholder.
> 
> Agreed.
> 
> 
> >>One question that might help resolve this: Would having some sort of
> >>volatility checking interface be helpful in easing your concern
> >>about applications being surprised by volatility?
> >If we can provide above things, I think we don't need such interface
> >until someone want it with reasonable logic.
> 
> Sure, I just wanted to know if you saw a need right away. For now we
> can leave it be.
> 
> >>True. And performance needs to be good if this hinting interface is
> >>to be used easily. Although I worry about performance trumping sane
> >>semantics. So let me try to implement the desired behavior and we
> >>can measure the difference.
> >NP. But keep in mind that mmap_sem was really terrible for performance
> >when I took a expereiment(ie, concurrent page fault by many threads
> >while a thread calls mmap).
> >I guess primary reason is CONFIG_MUTEX_SPIN_ON_OWNER.
> >So at least, we should avoid it by introducing new mode like
> >VOLATILE_ANON|VOLATILE_FILE|VOLATILE_BOTH if we want to
> >support mvrange-file and mvragne interface was thing userland people
> >really want although ashmem have used fd-based model.
> 
> The VOLATILE_ANON|VOLATILE_FILE|VOLATILE_BOTH may be an interesting
> compromise.
> 
> Though, if one marks a VOLATILE_ANON range on an address that's an
> mmaped file, how do we detect this and provide a sane error value
> without checking the vmas?
> 

Should we check vma?
If there are conflict with existing vrange type, just return an -EINVAL?

> 
> thanks
> -john
> 
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-- 
Kind regards,
Minchan Kim

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