Re: [RFC] Reserve huge pages for reliable MAP_PRIVATE hugetlbfs mappings

[Mel Gorman <mel@csn.ul.ie>]

On (23/04/08 17:43), Andi Kleen didst pronounce:
> > If the large pages exist to satisfy the mapping, the application will not
> > even notice this change. They will only break if the are creating larger
> > mappings than large pages exist for (or can be allocated for in the event
> > they have enabled dynamic resizing with nr_overcommit_hugepages). If they
> > are doing that, they are running a big risk as they may get arbitrarily
> > killed later. 
> 
> The point is it is pretty common (especially when you have enough 
> address space) just create a huge mapping and only use the begining.

Clearly these applications are assuming that overcommit is always allowed
because otherwise they would be failing. Also, the behaviour with reservations
is currently inconsistent as MAP_SHARED always tries to reserve the pages.
Maybe large shared mappings are rarer than large private mappings, but
this inconsistency is still shoddy.

> This avoids costly resize operations later and is a quite useful
> strategy on 64bit (but even on 32bit).  Now the upper size will
> likely be incredibly huge (far beyond available physical memory), but it's 
> obviously impossible really uses all of it.
> 
> It's also common in languages who don't support dynamic allocation well (like 
> older fortran dialects). Given these won't use hugetlbfs directly either, 
> but I couldn't rule out that someone wrote a special fortran run time library 
> which transparently allocates large arrays from hugetlbfs. 
> 
> In fact i would be surprised if a number of such beasts don't exist -- it is 
> really an obvious simple tuning option for old HPC fortran applications.
> 

I don't know if such a run-time library exists but libhugetlbfs is occasionally
used in situations like this to back the data segment backed by huge pages. It
first copies the data MAP_SHARED to fake the reservation before remapping
private and kinda hopes for the best.

> > Sometimes their app will run, other times it dies. If more
> > than one application is running on the system that is behaving like this,
> > they are really playing with fire.
> 
> With your change such an application will not run at all. Doesn't
> seem like an improvement to me.
> 

I disagree. mmap() failing is something an application should be able to
take care of and at least it possible as opposed to SIGKILL where it's
"game over". Even if they are using run-time helpers like libhugetlbfs,
it should be able to detect when a large-page-backed is flaky and use small
pages instead. As it is, it's very difficult to detect in advance if future
faults will succeed, particularly when more than one application is running.

If the application is unable to handle mmap() failing, then yes, it'll
exit. But at least it'll exit consistently and not get SIGKILLed
randomly which to me is a far worse situation.

> > With this change, a mmap() failure is a clear indication that the mapping
> > would have been unsafe to use and they should try mmap()ing with small pages
> > instead. 
> 
> I don't have a problem with having an optional strict overcommit checking 
> mode (similar to what standard VM has), but it should be configurable
> and off by default.
> 

There already is partial strict overcommit checking for MAP_SHARED. I
think it should be consistent and done for MAP_PRIVATE.

For disabling, I could look into adding MAP_NORESERVE support and
something like the current overcommit_memory tunable for hugetlbfs.
However with yet another proc tunable, I would push for enabling strict
checking by default for safe behaviour than disabled by default for
random SIGKILLs.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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