Re: [RFC v7 00/11] Support vrange for anonymous page

[Minchan Kim]

On Thu, Apr 11, 2013 at 04:15:40AM -0400, KOSAKI Motohiro wrote:
> (4/11/13 4:02 AM), Minchan Kim wrote:
> > On Thu, Apr 11, 2013 at 03:20:30AM -0400, KOSAKI Motohiro wrote:
> >>>>>   DONTNEED makes sure user always can see zero-fill pages after
> >>>>>   he calls madvise while vrange can see data or encounter SIGBUS.
> >>>>
> >>>> For replacing DONTNEED, user want to zero-fill pages like DONTNEED
> >>>> instead of SIGBUS. So, new flag option would be nice.
> >>>
> >>> If userspace people want it, I can do it. 
> >>> But not sure they want it at the moment becaue vrange is rather
> >>> different concept of madvise(DONTNEED) POV usage.
> >>>
> >>> As you know well, in case of DONTNEED, user calls madvise _once_ and
> >>> VM releases memory as soon as he called system call.
> >>> But vrange is same with delayed free when the system memory pressure
> >>> happens so user can't know OS frees the pages anytime.
> >>> It means user should call pair of system call both VRANGE_VOLATILE
> >>> and VRANGE_NOVOLATILE for right usage of volatile range
> >>> (for simple, I don't want to tell SIGBUS fault recovery method).
> >>> If he took a mistake(ie, NOT to call VRANGE_NOVOLATILE) on the range
> >>> which is used by current process, pages used by some process could be
> >>> disappeared suddenly.
> >>>
> >>> In summary, I don't think vrange is a replacement of madvise(DONTNEED)
> >>> but could be useful with madvise(DONTNEED) friend. For example, we can
> >>> make return 1 in vrange(VRANGE_VOLATILE) if memory pressure was already
> >>
> >> Do you mean vrange(VRANGE_UNVOLATILE)?
> > 
> > I meant VRANGE_VOLATILE. It seems my explanation was poor. Here it goes, again.
> > Now vrange's semantic return just 0 if the system call is successful, otherwise,
> > return error. But we can change it as folows
> > 
> > 1. return 0 if the system call is successful and memory pressure isn't severe
> > 2. return 1 if the system call is successful and memory pressure is severe
> > 3. return -ERRXXX if the system call is failed by some reason
> > 
> > So the process can know system-wide memory pressure without peeking the vmstat
> > and then call madvise(DONTNEED) right after vrange call. The benefit is system
> > can zap all pages instantly.
> 
> Do you mean your patchset is not latest? and when do you use this feature? what's

Yes. I meant I can it in next spin up for hearing the opinion.

> happen VRANGE_VOLATILE return 0 and purge the range just after returning syscall.

It could be an idea but I will think it over.

> 
> 
> >> btw, assign new error number to asm-generic/errno.h is better than strange '1'.
> > 
> > I can and admit "1" is rather weired.
> > But it's not error, either.
> 
> If this is really necessary, I don't oppose it. However I am still not convinced.
> 
> 
> 
> >>> severe so user can catch up memory pressure by return value and calls
> >>> madvise(DONTNEED) if memory pressure was already severe. Of course, we
> >>> can handle it vrange system call itself(ex, change vrange system call to
> >>> madvise(DONTNEED) but don't want it because I want to keep vrange hinting
> >>> sytem call very light at all times so user can expect latency.
> >>
> >> For allocator usage, vrange(UNVOLATILE) is annoying and don't need at all.
> >> When data has already been purged, just return new zero filled page. so,
> >> maybe adding new flag is worthwhile. Because malloc is definitely fast path
> > 
> > I really want it and it's exactly same with madvise(MADV_FREE).
> > But for implementation, we need page granularity someting in address range
> > in system call context like zap_pte_range(ex, clear page table bits and
> > mark something to page flags for reclaimer to detect it).
> > It means vrange system call is still bigger although we are able to remove
> > lazy page fault.
> > 
> > Do you have any idea to remove it? If so, I'm very open to implement it.
> 
> Hm. Maybe I am missing something. I'll look the code closely after LFS.

Please see the Rik's old work about MADV_FREE.

> 
> 
> >> and adding new syscall invokation is unwelcome.
> > 
> > Sure. But one more system call could be cheaper than page-granuarity
> > operation on purged range.
> 
> I don't think vrange(VOLATILE) cost is the related of this discusstion.
> Whether sending SIGBUS or just nuke pte, purge should be done on vmscan,
> not vrange() syscall.

Again, please see the MADV_FREE. http://lwn.net/Articles/230799/
It does changes pte and page flags on all pages of the range through
zap_pte_range. So it would make vrange(VOLASTILE) expensive and
the bigger cost is, the bigger range is.

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-- 
Kind regards,
Minchan Kim
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