Re: [PATCH 2/4] MADV_VOLATILE: Add MADV_VOLATILE/NONVOLATILE hooks and handle marking vmas

Minchan Kim
2014-05-09T00:05:48Z

On Thu, May 08, 2014 at 04:43:07PM -0700, John Stultz wrote:
> On 05/08/2014 04:12 PM, Minchan Kim wrote:
> > On Thu, May 08, 2014 at 09:38:40AM -0700, John Stultz wrote:
> >> On 05/07/2014 06:21 PM, Minchan Kim wrote:
> >>> Hey John,
> >>>
> >>> On Tue, Apr 29, 2014 at 02:21:21PM -0700, John Stultz wrote:
> >>>> This patch introduces MADV_VOLATILE/NONVOLATILE flags to madvise(),
> >>>> which allows for specifying ranges of memory as volatile, and able
> >>>> to be discarded by the system.
> >>>>
> >>>> This initial patch simply adds flag handling to madvise, and the
> >>>> vma handling, splitting and merging the vmas as needed, and marking
> >>>> them with VM_VOLATILE.
> >>>>
> >>>> No purging or discarding of volatile ranges is done at this point.
> >>>>
> >>>> This a simplified implementation which reuses some of the logic
> >>>> from Minchan's earlier efforts. So credit to Minchan for his work.
> >>> Remove purged argument is really good thing but I'm not sure merging
> >>> the feature into madvise syscall is good idea.
> >>> My concern is how we support user who don't want SIGBUS.
> >>> I believe we should support them because someuser(ex, sanitizer) really
> >>> want to avoid MADV_NONVOLATILE call right before overwriting their cache
> >>> (ex, If there was purged page for cyclic cache, user should call NONVOLATILE
> >>> right before overwriting to avoid SIGBUS).
> >> So... Why not use MADV_FREE then for this case?
> > MADV_FREE is one-shot operation. I mean we should call it again to make
> > them lazyfree while vrange could preserve volatility.
> > Pz, think about thread-sanitizer usecase. They do mmap 70TB once start up
> > and want to mark the range as volatile. If they uses MADV_FREE instead of
> > volatile, they should mark 70TB as lazyfree periodically, which is terrible
> > because MADV_FREE's cost is O(N).
>
> I still have had difficulty seeing the thread-sanitizer usage as a
> generic enough model for other applications. I realize they want to
> avoid marking and unmarking ranges (and they want that marking and
> unmarking to be very cheap), but the zero-fill purged page (while still
> preserving volatility) causes lots of *very* strange behavior:

I don't think it's for only thread-sanitizer.
Pz, think following usecase.

Let's assume big volatile cache.
If there is request for cache, it should find a object in a cache
and if it found, it should call vrange(NOVOLATILE) right before
passing it to the user and investigate it was purged or not.
If it wasn't purged, cache manager could pass the object to the user.
But it's circular cache so if there is no request from user, cache manager
always overwrites objects so it could encounter SIGBUS easily
so as current sematic, cache manager always should call vrange(NOVOLATILE)
right before the overwriting. Otherwise, it should register SIGBUS handler
to unmark volatile by page unit. SIGH.

If we support zero-fill, cache manager could overwrite object without
SIGBUS handling or vrange(NOVOLATILE) call right before overwriting.
Just what we need is vrange(NOVOLATILE) call right before passing it
to user.

>
> * How do general applications know the difference between a purged page
> and a valid empty page?
> * When reading/writing a page, what happens if half-way the application
> is preempted, and the page is purged?
> * If a volatile page is purged, then zero-filled on a read or write,
> what is its purged state when we're marking it non-volatile?

Maybe above scenario goes your questions to VOID.

>
> These use cases don't seem completely baked, or maybe I've just not been
> able to comprehend them yet. But I don't quite understand the desire to
> prioritize this style of usage over other simpler and more well
> established usage?

I think it's one of typical usecase of vrange syscall.

>
> I'll grant that there may be some form of semantics that work for this,
> and I'm open to considering support for those at some point if they
> become more clear, but I don't think these stranger(to me at least)
> cases should be the default, and I really worry that these requests
> continue to make the basic usage harder to understand for reviewers.
>
>
> >> Just to be clear, by moving back to madvise, I'm not trying to replace
> >> MADV_FREE. I think you're work there is still useful and splitting the
> >> semantics between the two is cleaner.
> > I know.
> > New vrange syscall which works with existing VMA instead of new vrange
> > interval tree removed big concern from mm folks about duplicating
> > of manage layer(ex, vm_area_struct and vrange inteval tree) and
> > it removed my concern that mmap_sem write-side lock scalability for
> > allocator usecase so we can make the implemenation simple and clear.
> > I like it but zero-page VS SIGBUS is another issue we should make an
> > agreement.
>
> Zero-fill makes sense to me for MADV_FREE, where we're not trying to
> recover the data, but just save the cost of releasing and re-faulting
> possibly frequently used pages. The contents are not intended to be
> recovered. Thus semantics there are reasonable.
>
> With volatility (which persists until marked non-volatile), zero-filled
> purged page access breaks quite a bit of the established semantics (see
> the strange behavior questions listed above).
>
> With SIGBUS semantics, its very clear and much more simple. The
> application has hit an page that no longer exists and is clearly
> notified (via SIGBUS). There's no way for the purged state to become
> lost (other then the application ignoring the return value from
> MADV_NONVOLATILE).
>
> Again, I do understand that folks want a solution to the
> thread-sanitizer usage model, but I really think, much as we found with
> MADV_FREE, that its really a quite different semantics that are wanted,
> and trying to mix them doesn't help get anything reviewed/merged.
>
> thanks
> -john
>
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--
Kind regards,
Minchan Kim