Re: [RFC][PATCH] hugetlb: add information and interface in sysfs [Was Re: [RFC][PATCH 4/5] Documentation: add node files to sysfs ABI]

[Nishanth Aravamudan <nacc@us.ibm.com>]

On 28.04.2008 [13:31:00 -0700], Christoph Lameter wrote:
> On Sat, 26 Apr 2008, Greg KH wrote:
> 
> > Also, why use a "units" here, just always use the lowest unit, and
> > userspace can convert from kB to GB if needed.
> 
> Additional complications will come about because IA64 supports 
> varying hugetlb sizes from 4kb to 1GB.

What "complications" do you mean? It's a small function indeed to
convert from the directory name to the corresponding "human-named" size,
e.g. hugepages-1048576 to "1 GB". And such a function will probably
exist in libhugetlbfs at some point, for applications to use, if they
like.

A potential problem I do see is for a 32-bit binary running on a 64-bit
kernel and is one we've run against for 32-bit binaries with 16G pages
available. The 32-bit binary can't actually store the size of the
hugepage in an unsigned long, so we have to remember how big of a value
we can represent (i.e., max_hugepage_size_in_kb) and check what's
obtained from /proc/meminfo against that. Not ideal, for sure.

> Also we would at some point like to add support for 1TB hugepages
> (that may depend on the presence of a special device that handles
> these).

I also don't see a limitation here? For 32-bit programs, we'll see
1073741824 and know we can't convert that into a valid value in bytes.

More importnatly, I think the fact that IA64 supports multiple hugepage
sizes is a reason *for* moving to sysfs for this information? However, I
think we may need to massage the IA64-specific bits of the kernel to
actually support multiple hugepage size pools being available at
run-time? That is, with the current kernel, we can only support one
hugepagesize at run-time, due to VHPT restrictions?

Thanks,
Nish

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