Re: [PATCH] mm: fix negative commitlimit when gigantic hugepages are allocated

[Rafael Aquini <aquini@linux.com>]

Howdy Andrew,

Sorry, for this late reply.

On Thu, Jun 09, 2011 at 04:44:08PM -0700, Andrew Morton wrote:
> On Thu, 2 Jun 2011 23:55:57 -0300
> Rafael Aquini <aquini@linux.com> wrote:
> 
> > When 1GB hugepages are allocated on a system, free(1) reports
> > less available memory than what really is installed in the box.
> > Also, if the total size of hugepages allocated on a system is
> > over half of the total memory size, CommitLimit becomes
> > a negative number.
> > 
> > The problem is that gigantic hugepages (order > MAX_ORDER)
> > can only be allocated at boot with bootmem, thus its frames
> > are not accounted to 'totalram_pages'. However,  they are
> > accounted to hugetlb_total_pages()
> > 
> > What happens to turn CommitLimit into a negative number
> > is this calculation, in fs/proc/meminfo.c:
> > 
> >         allowed = ((totalram_pages - hugetlb_total_pages())
> >                 * sysctl_overcommit_ratio / 100) + total_swap_pages;
> > 
> > A similar calculation occurs in __vm_enough_memory() in mm/mmap.c.
> > 
> > Also, every vm statistic which depends on 'totalram_pages' will render
> > confusing values, as if system were 'missing' some part of its memory.
> 
> Is this bug serious enough to justify backporting the fix into -stable
> kernels?

Despite not having testing it, I can think the following scenario as
troublesome:
When gigantic hugepages are allocated and sysctl_overcommit_memory == OVERCOMMIT_NEVER.
In a such situation, __vm_enough_memory() goes through the mentioned 'allowed'
calculation and might end up mistakenly returning -ENOMEM, thus forcing
the system to start reclaiming pages earlier than it would be ususal, and this could
cause detrimental impact to overall system's performance, depending on the
workload.

Besides the aforementioned scenario, I can only think of this causing annoyances
with memory reports from /proc/meminfo and free(1).

Thanks for your attention!

Cheers!
-- 
Rafael Aquini <aquini@linux.com>

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