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?
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