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

From: John Stultz
Date: Tue Apr 09 2013 - 18:36:29 EST

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.

Hopefully that calms your worries here. :)

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
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.

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.


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
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?


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