[ Sorry, my responsiveness is horrible these days... ]
On Mon, Apr 23, 2012 at 04:15:06PM -0700, David Rientjes wrote:
> On Wed, 11 Apr 2012, Johannes Weiner wrote:
>
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -950,6 +950,8 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
> > > count_vm_event(THP_FAULT_FALLBACK);
> > > ret = do_huge_pmd_wp_page_fallback(mm, vma, address,
> > > pmd, orig_pmd, page, haddr);
> > > + if (ret & VM_FAULT_OOM)
> > > + split_huge_page(page);
> > > put_page(page);
> > > goto out;
> > > }
> > > @@ -957,6 +959,7 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
> > >
> > > if (unlikely(mem_cgroup_newpage_charge(new_page, mm, GFP_KERNEL))) {
> > > put_page(new_page);
> > > + split_huge_page(page);
> > > put_page(page);
> > > ret |= VM_FAULT_OOM;
> > > goto out;
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -3489,6 +3489,7 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> > > if (unlikely(is_vm_hugetlb_page(vma)))
> > > return hugetlb_fault(mm, vma, address, flags);
> > >
> > > +retry:
> > > pgd = pgd_offset(mm, address);
> > > pud = pud_alloc(mm, pgd, address);
> > > if (!pud)
> > > @@ -3502,13 +3503,24 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> > > pmd, flags);
> > > } else {
> > > pmd_t orig_pmd = *pmd;
> > > + int ret;
> > > +
> > > barrier();
> > > if (pmd_trans_huge(orig_pmd)) {
> > > if (flags & FAULT_FLAG_WRITE &&
> > > !pmd_write(orig_pmd) &&
> > > - !pmd_trans_splitting(orig_pmd))
> > > - return do_huge_pmd_wp_page(mm, vma, address,
> > > - pmd, orig_pmd);
> > > + !pmd_trans_splitting(orig_pmd)) {
> > > + ret = do_huge_pmd_wp_page(mm, vma, address, pmd,
> > > + orig_pmd);
> > > + /*
> > > + * If COW results in an oom, the huge pmd will
> > > + * have been split, so retry the fault on the
> > > + * pte for a smaller charge.
> > > + */
> > > + if (unlikely(ret & VM_FAULT_OOM))
> > > + goto retry;
> >
> > Can you instead put a __split_huge_page_pmd(mm, pmd) here? It has to
> > redo the get-page-ref-through-pagetable dance, but it's more robust
> > and obvious than splitting the COW page before returning OOM in the
> > thp wp handler.
>
> I agree it's more robust if do_huge_pmd_wp_page() were modified later and
> mistakenly returned VM_FAULT_OOM without the page being split, but
> __split_huge_page_pmd() has the drawback of also requiring to retake
> mm->page_table_lock to test whether orig_pmd is still legitimate so it
> will be slower. Do you feel strongly about the way it's currently written
> which will be faster at runtime?
If you can't accomodate for a hugepage, this code runs 511 times in
the worst case before you also can't fit a regular page anymore. And
compare it to the cost of the splitting itself and the subsequent 4k
COW break faults...
I don't think it's a path worth optimizing for at all, especially if
it includes sprinkling undocumented split_huge_pages around, and the
fix could be as self-contained as something like this...
diff --git a/mm/memory.c b/mm/memory.c
index 706a274..dae0afc 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3505,14 +3505,29 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
pmd, flags);
} else {
pmd_t orig_pmd = *pmd;
+ int ret;
+
barrier();
if (pmd_trans_huge(orig_pmd)) {
if (flags & FAULT_FLAG_WRITE &&
!pmd_write(orig_pmd) &&
- !pmd_trans_splitting(orig_pmd))
- return do_huge_pmd_wp_page(mm, vma, address,
+ !pmd_trans_splitting(orig_pmd)) {
+ ret = do_huge_pmd_wp_page(mm, vma, address,
pmd, orig_pmd);
- return 0;
+ if (unlikely(ret & VM_FAULT_OOM)) {
+ /*
+ * It's not worth going OOM
+ * over not being able to
+ * allocate or charge a full
+ * copy of the huge page.
+ * Split it up and handle as
+ * single page COW break below.
+ */
+ __split_huge_page_pmd(mm, pmd);
+ } else
+ return ret;
+ } else
+ return 0;
}
}
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>