On Mon, Apr 09, 2012 at 10:42:31PM -0700, David Rientjes wrote:
> On COW, a new hugepage is allocated and charged to the memcg. If the
> system is oom or the charge to the memcg fails, however, the fault
> handler will return VM_FAULT_OOM which results in an oom kill.
>
> Instead, it's possible to fallback to splitting the hugepage so that the
> COW results only in an order-0 page being allocated and charged to the
> memcg which has a higher liklihood to succeed. This is expensive because
> the hugepage must be split in the page fault handler, but it is much
> better than unnecessarily oom killing a process.
>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> mm/huge_memory.c | 3 +++
> mm/memory.c | 18 +++++++++++++++---
> 2 files changed, 18 insertions(+), 3 deletions(-)
>
> 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.
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