linux-kernel - Re: [PATCH] [BUGFIX] mm: hugepages can cause negative commitlimit

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Date:	Wed, 1 Jun 2011 23:08:31 -0500
From:	Russ Anderson <rja@....com>
To:	Rafael Aquini <aquini@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>, rja@...ricas.sgi.com
Subject: Re: [PATCH] [BUGFIX] mm: hugepages can cause negative commitlimit

On Thu, May 26, 2011 at 06:07:53PM -0300, Rafael Aquini wrote:
> On Fri, May 20, 2011 at 07:30:32PM -0300, Rafael Aquini wrote:
> > On Fri, May 20, 2011 at 01:04:11PM -0700, Andrew Morton wrote:
> > > On Thu, 19 May 2011 17:11:01 -0500
> > > Russ Anderson <rja@....com> wrote:
> > > 
> > > > OK, I see your point.  The root problem is hugepages allocated at boot are
> > > > subtracted from totalram_pages but hugepages allocated at run time are not.
> > > > Correct me if I've mistate it or are other conditions.
> > > > 
> > > > By "allocated at run time" I mean "echo 1 > /proc/sys/vm/nr_hugepages".
> > > > That allocation will not change totalram_pages but will change
> > > > hugetlb_total_pages().
> > > > 
> > > > How best to fix this inconsistency?  Should totalram_pages include or exclude
> > > > hugepages?  What are the implications?
> > > 
> > > The problem is that hugetlb_total_pages() is trying to account for two
> > > different things, while totalram_pages accounts for only one of those
> > > things, yes?
> > > 
> > > One fix would be to stop accounting for huge pages in totalram_pages
> > > altogether.  That might break other things so careful checking would be
> > > needed.
> > > 
> > > Or we stop accounting for the boot-time allocated huge pages in
> > > hugetlb_total_pages().  Split the two things apart altogether and
> > > account for boot-time allocated and runtime-allocated pages separately.  This
> > > souds saner to me - it reflects what's actually happening in the kernel.
> > 
> > Perhaps we can just reinstate the # of pages "stealed" at early boot allocation
> > later, when hugetlb_init() calls gather_bootmem_prealloc()
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 8ee3bd8..d606c9c 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1111,6 +1111,7 @@ static void __init gather_bootmem_prealloc(void)
> >                 WARN_ON(page_count(page) != 1);
> >                 prep_compound_huge_page(page, h->order);
> >                 prep_new_huge_page(h, page, page_to_nid(page));
> > +               totalram_pages += 1 << h->order;
> >         }
> >  }
> 
> Howdy Russ,
> 
> Were you able to confirm if that proposed change fix the issue you've reported?

Yes, it fixes the inconsistency in reporting totalram_pages.

> Although I've tested it with usual size hugepages and it did not messed things up,
> I'm not able to test it with GB hugepages, as I do not have any proc with "pdpe1gb" flag available.

There seems to be another issue.  1G hugepages can be allocated at boot time, but
cannot be allocated at run time.  "default_hugepagesz=1G hugepagesz=1G hugepages=1" on 
the boot line works.  With "default_hugepagesz=1G hugepagesz=1G" the command
"echo 1 > /proc/sys/vm/nr_hugepages" fails.

uv4-sys:~ # echo 1 > /proc/sys/vm/nr_hugepages
-bash: echo: write error: Invalid argument


> Thanks in advance!
> Cheers!
> -- 
> Rafael Aquini <aquini@...ux.com>

-- 
Russ Anderson, OS RAS/Partitioning Project Lead  
SGI - Silicon Graphics Inc          rja@....com
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