Re: RFC: Cleanup / small fixes to hugetlb fault handling

David Gibson
2005-10-27T00:20:51Z

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

> > - 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>
>
> > +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.