Re: [PATCH 0/2] [RFC] Volatile Ranges (v6)

[John Stultz]

On 04/07/2012 01:14 AM, Dmitry Adamushko wrote:
> On 7 April 2012 02:08, John Stultz<john.stultz@xxxxxxxxxx>  wrote:
> > Another detail is that by hanging the volatile ranges off of the
> > address_space, the volatility for tmpfs files persists even when no one
> > has an open fd on the file. This could cause some surprises if application
> > A marked some pages volatile and died, then application B opened the file
> > and had pages dropped out underneith it while it was being used. I suspect
> > I need to clean up the volatility when all fds are dropped.
> And how do you handle the regions that have already been purged by
> this moment? Unless B has some specific mechanism to verify the
> consistency of the content, a sensible way would be to always mark off
> the regions as non-volatile before accessing them and verify the
> return code to see if there are holes.
>
> More generally, what if B opens the file while A is still working with
> it? Besides the use of normal synchronization mechanisms, B should not
> make any assumption on the current state of the regions (unless there
> is a high-level protocol between A and B to share this info). So an
> explicit mark-off-as-non_volatile could be a simple generic mechanism.

So yes, marking as non-volatile before you use pages would be a way to 
avoid the issue.  But it still rubs me the wrong way.

I think the main issue I have with it is that it makes volatility the 
assumed state. So unless you mark it non-volatile to begin with, the 
file could be volatile somewhere.  I feel like volatility should be the 
special state, not the assumed one, so normal applications that don't 
think about volatility are less-likely to be surprised.

Now, when you have concurrent users of a file, you have to coordinate, 
and things can change under you. That's an expectation people already 
have.  But if volatile ranges persist, its sort of introducing a form of 
concurrency to non-concurrent access.  Where a killed application can 
reach from the grave and zap a page in file someone else is using.  I 
think this is too unexpected.

The case that bit me in particular was in testing this patch, I had an 
application (call it A) that had a bug and was marking a larger range 
volatile then it re-set to non-volatile.   Then when using the same file 
later with a different test application (call it B), I was seeing those 
further pages be zapped unexpectedly.  It took me a while to realize 
that it wasn't a problem with the B application, or the patch itself, 
but was a persistent range that was set much earlier by A.

So I suspect it would be better if the volatile ranges should be 
something that are cleared out when the last fd is closed.

thanks
-john




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