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

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.

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?

quoted

+#define HPAGE_INACTIVE  0
+#define HPAGE_ACTIVE    1

The above need documenting, please.  That documentation would perhaps
help me understand why we need both an "active" flag *and* a refcount.

It doesn't need both. Now that you mention it, it would be simpler to put it all
in the refcount. I'd send an updated patch, but it sounds like things will be
going in a different direction.


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