On Wed, Oct 26, 2005 at 04:46:16PM -0500, Adam Litke wrote:

On Wed, 2005-10-26 at 12:48 +1000, David Gibson wrote:

quoted

On Wed, Oct 26, 2005 at 12:00:55PM +1000, David Gibson wrote:

quoted

Hi, Adam, Bill, Hugh,

Does this look like a reasonable patch to send to akpm for -mm.

Ahem.  Or rather this version, which actually compiles.

This patch makes some slight tweaks / cleanups to the fault handling
path for huge pages in -mm.  My main motivation is to make it simpler
to fit COW in, but along the way it addresses a few minor problems
with the existing code:

- The check against i_size was duplicated: once in
  find_lock_huge_page() and again in hugetlb_fault() after taking the
  page_table_lock.  We only really need the locked one, so remove the
  other.

Fair enough.

quoted

- find_lock_huge_page() didn't, in fact, lock the page if it newly
  allocated one, rather than finding it in the page cache already.  As
  far as I can tell this is a bug, so the patch corrects it.

Thanks.  I was about to post a fix for this too.  It is reproducible in
the case where two threads race in the fault handler and both do
alloc_huge_page().  In that case, the loser will fail to insert his page
into the page cache and will call put_page() which has a
BUG_ON(page_count(page) == 0).

Hrm.. I don't think this is due to that.  As Ken points out,
add_to_page_cache() does indeed lock the page if it suceeds in
inserting.  And alloc_huge_page() sets the count to 1, not 0, so the
put_page() should not cause a problem on the failure path.

Hrm.. wonder what's going on here.

quoted

- find_lock_huge_page() isn't a great name, since it does extra things
  not analagous to find_lock_page().  Rename it
  find_or_alloc_huge_page() which is closer to the mark.

I'll agree with the above.  I am not all that committed to the current
layout and what you have here is a little closer to the thinking in my
original patch ;)

<snip>

quoted

+int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
+		  unsigned long address, int write_access)
+{
+	pte_t *ptep;
+	pte_t entry;
+
+	ptep = huge_pte_alloc(mm, address);
+	if (! ptep)
+		/* OOM */
+		return VM_FAULT_SIGBUS;
+
+	entry = *ptep;
+
+	if (pte_none(entry))
+		return hugetlb_no_page(mm, vma, address, ptep);
+
+	/* we could get here if another thread instantiated the pte
+	 * before the test above */
+
+	return VM_FAULT_SIGBUS;
 }

I'll agree with Ken that the last return should probably still be
VM_FAULT_MINOR.

-- 
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/people/dgibson

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