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

From: John Stultz
Date: Mon Apr 08 2013 - 23:28:00 EST

On 04/08/2013 07:18 PM, Minchan Kim wrote:
On Mon, Apr 08, 2013 at 05:36:42PM -0700, John Stultz wrote:
On 04/07/2013 05:46 PM, Minchan Kim wrote:
Hello John,

As you know, userland people wanted to handle vrange with mmaped
pointer rather than fd-based and see the SIGBUS so I thought more
about semantic of vrange and want to make it very clear and easy.
So I suggest below semantic(Of course, it's not rock solid).

mvrange(start_addr, lengh, mode, behavior)

It's same with that I suggested lately but different name, just
adding prefix "m". It's per-process model(ie, mm_struct vrange)
so if process is exited, "volatility" isn't valid any more.
It isn't a problem in anonymous but could be in file-vrange so let's
introduce fvrange for covering the problem.

fvrange(int fd, start_offset, length, mode, behavior)

First of all, let's see mvrange with anonymous and file page POV.

1) anon-mvrange

The page in volaitle range will be purged only if all of processes
marked the range as volatile.

If A process calls mvrange and is forked, vrange could be copied
>from parent to child so not-yet-COWed pages could be purged
unless either one of both processes marks NO_VOLATILE explicitly.

Of course, COWed page could be purged easily because there is no link
any more.
Ack. This seems reasonable.

2) file-mvrange

A page in volatile range will be purged only if all of processes mapped
the page marked it as volatile AND there is no process mapped the page
as "private". IOW, all of the process mapped the page should map it
with "shared" for purging.

So, all of processes should mark each address range in own process
context if they want to collaborate with shared mapped file and gaurantee
there is no process mapped the range with "private".

Of course, volatility state will be terminated as the process is gone.
This case doesn't seem ideal to me, but is sort of how the current
code works to avoid the complexity of dealing with memory volatile
ranges that cross page types (file/anonymous). Although the current
code just doesn't purge file pages marked with mvrange().
Personally, I don't think it's to avoid the complexity of implemenation.
I thought explict declaration volatility on range before using would be
more clear for userspace programmer.
Otherwise, he can encounter SIGBUS and got confused easily.

Frankly speaking, I don't like to remain volatility permanently although
relavant processes go away and it could make processs using the file
much error-prone and hard to debug it.

So this is maybe is a contentious point we'll have to work out.

Maybe could you describe some use cases you envision where someone would want to mark pages volatile on a file that could be accidentally shared? Or how you think the per-mm sense of volatility would be beneficial in those use-cases?

The use cases I envision where volatility would be used are when any sharing would be coordinated between processes.
Again, that producer/consumer example from before where the empty portion of a very large circular buffer could be made volatile, scaling the actual memory usage to the actual need.

And really the same concern would likely apply in the common case when multiple applications mmap (shared) a file, but use fvrange() to mark the data as volatile. This is exactly the use case the Android ashmem interface works for. In that case, once the data is 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.

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.

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).

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?

Anyway, do you agree my suggestion that "we should not purge any page if
a process are using now with non-shared(ie, private)"?

Yes, or if we do purge any pages, they should not affect the private mapped pages (in other words, the COW link should be broken - as the backing page has in-effect been written to by purging).

I'd much prefer file-mvrange calls to behave identically to fvrange calls.

The important point here is that the kernel doesn't *have* to purge
anything ever. Its the kernel's discretion as to which volatile
pages to purge when. So its easier for now to simply not purge file

pages marked volatile via mvolatile.
NP but we should write down vague description. User try to use it
in file-backed pages and got disappointed, then is reluctant to use it
any more. :)

I'm not saying that let's write down description implementation specific
but want to say them at least new system call can affect anonymous or file
or both, at least from the beginning. Just hope.

I'd like to make it generic enough that we have some flexibility to modify the puring rules if we find its more optimal. But I agree, the desired semantics of what could occur should be clear.

There however is the inconsistency that file pages marked volatile
via fvrange, then are marked non-volatile via mvrange() might still
be purged. That is broken in my mind, and still needs to be
addressed. The easiest out is probably just to return an error if
any of the mvrange calls cover file pages. But I'd really like a
It needs vma enumeration and mmap_sem read-lock.
It could hurt anon-vrange performance severely.

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.

better fix.
Another idea is that we can move per-mm vrange element to address_space
when the process goes away if the element covers file-backd vma.
But I'm still very not sure whether we should keep it persistent.

I really think the persistence of file-backed volatile ranges (as long as someone has the file open or a mapping to it) is important. Again, I think of the volatility really being a state of the page, but since a page-based approach is too costly, we're optimizing it into mm_struct state or address_space state.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at