Messages in this thread | | | Date | Mon, 09 Apr 2012 10:56:39 -0700 | From | John Stultz <> | Subject | Re: [PATCH 0/2] [RFC] Volatile Ranges (v6) |
| |
On 04/07/2012 01:14 AM, Dmitry Adamushko wrote: > On 7 April 2012 02:08, John Stultz<john.stultz@linaro.org> 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
| |