Re: [PATCH v3 13/14] mm, hugetlb: retry if failed to allocate andthere is concurrent user

[Andrew Morton]

On Fri, 20 Dec 2013 10:58:10 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:

> On Thu, Dec 19, 2013 at 05:02:02PM -0800, Andrew Morton wrote:
> > On Wed, 18 Dec 2013 15:53:59 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> > 
> > > If parallel fault occur, we can fail to allocate a hugepage,
> > > because many threads dequeue a hugepage to handle a fault of same address.
> > > This makes reserved pool shortage just for a little while and this cause
> > > faulting thread who can get hugepages to get a SIGBUS signal.
> > > 
> > 
> > So if I'm understanding this correctly...  if N threads all generate a
> > fault against the same address, they will all dive in and allocate a
> > hugepage, will then do an enormous memcpy into that page and will then
> > attempt to instantiate the page in pagetables.  All threads except one
> > will lose the race and will free the page again!  This sounds terribly
> > inefficient; it would be useful to write a microbenchmark which
> > triggers this scenario so we can explore the impact.
> 
> Yes, you understand correctly, I think.
> 
> I have an idea to prevent this overhead. It is that marking page when it
> is zeroed and unmarking when it is mapped to page table. If page mapping
> is failed due to current thread, the zeroed page will keep the marker and
> later we can determine if it is zeroed or not.

Well OK, but the other threads will need to test that in-progress flag
and then do <something>.  Where <something> will involve some form of
open-coded sleep/wakeup thing.  To avoid all that wheel-reinventing we
can avoid using an internal flag and use an external flag instead. 
There's one in struct mutex!

I doubt if the additional complexity of the external flag is worth it,
but convincing performance testing results would sway me ;) Please have
a think about it all.

> If you want to include this functionality in this series, I can do it ;)
> Please let me know your decision.
> 
> > I'm wondering if a better solution to all of this would be to make
> > hugetlb_instantiation_mutex an array of, say, 1024 mutexes and index it
> > with a hash of the faulting address.  That will 99.9% solve the
> > performance issue which you believe exists without introducing this new
> > performance issue?
> 
> Yes, that approach would solve the performance issue.
> IIRC, you already suggested this idea roughly 6 months ago and it is
> implemented by Davidlohr. I remembered that there is a race issue on
> COW case with this approach. See following link for more information.
> https://lkml.org/lkml/2013/8/7/142

That seems to be unrelated to hugetlb_instantiation_mutex?

> And we need 1-3 patches to prevent other theorectical race issue
> regardless any approaches.

Yes, I'll be going through patches 1-12 very soon, thanks.


And to reiterate: I'm very uncomfortable mucking around with
performance patches when we have run no tests to measure their
magnitude, or even whether they are beneficial at all!
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