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

[Minchan Kim <minchan@kernel.org>]

On Thu, May 08, 2014 at 05:24:41PM -0700, John Stultz wrote:
> On 05/08/2014 05:07 PM, Minchan Kim wrote:
> > 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.
> 
> But that wouldn't work. If the page was purged half way through writing
> it, we end up with a page of half zero data and half written data. What
> would the page state be at that point? Purged? Not purged?

You're right. Application might detect it with adding a sentinel in the
header but I don't think it should be generic model with zero-fill semantic
although some of application could do it.

> 
> * If its not purged (since a write was done to the page after being
> zero-filled), we will silently return to the user corrupted data.
> 
> * If it is considered purged, how do we store that data? Since we
> currently detect purged pages by checking if they are present when we
> mark non-volatile.
> 
> 
> This sort of zero-fill behavior on volatile pages only seems to make
> sense if pages are written atomically.
> 
> The SIGBUS handling solution you SIGH'ed at above actually seems
> reasonable, because it would allow the page to be safely filled
> atomically (marking it non-volatile, filling it and then re-marking it
> volatile). Sure it would cost more, fast and wrong isn't really a valid
> option.

Got it. My scenario was totally broken so I don't insist on such model any more.
First of all, let's go with SIGBUS model first if there is no strong requirement
from user folks.

Thanks for pointing out, John!

> 
> 
> 
> >
> >> * 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.
> 
> I'm not sure I understand this.
> 
> 
> >
> >> 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 apologize if I'm seeming stubborn, but I just can't see how it would
> work sanely.
> 
> thanks
> -john
> 
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-- 
Kind regards,
Minchan Kim

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