On Mon, Aug 22, 2011 at 03:07:54PM -0500, Andrew Barry wrote:

On 08/19/2011 04:51 PM, Andrew Morton wrote:

quoted

What's different about hugetlbfs?  Why don't other filesystems hit this?

<investigates further>

OK so the incorrect interaction happened in free_huge_page(), which is
called via the compound page destructor (this dtor is "what's different
about hugetlbfs").   What is incorrect about this is

a) that we're doing fs operations in response to a
   get_user_pages()/put_page() operation which has *nothing* to do with
   filesystems!

b) that we continue to try to do that fs operation against an fs
   which was unmounted and freed three days ago. duh.

Yes.

quoted

So I hereby pronounce that

a) It was wrong to manipulate hugetlbfs quotas within
   free_huge_page().  Because free_huge_page() is a low-level
   page-management function which shouldn't know about one of its
   specific clients (in this case, hugetlbfs).

   In fact it's wrong for there to be *any* mention of hugetlbfs
   within hugetlb.c.

b) I shouldn't have merged that hugetlbfs quota code.  whodidthat. 
   Mel, Adam, Dave, at least...

c) The proper fix here is to get that hugetlbfs quota code out of
   free_huge_page() and do it all where it belongs: within hugetlbfs
   code.


Regular filesystems don't need to diddle quota counts within
page_cache_release().  Why should hugetlbfs need to?

Is there anyone, more expert in hugetlbfs code than I, who can/should/will take
that on?

As far as I can tell the hugetlbfs "quota" counts that are updated
here don't share much with the normal quota mechanisms.  The way they
operate, they logically divide the pool of free huge pages between
different hugetlbfs instances.  This means that you can give different
hugepage mounts to different applications and they won't be able to
exhaust each others resources.

I can't see how that can be done without updating the count somewhere
at free_huge_page() time.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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