Re: [patch] thp, memcg: split hugepage for memcg oom on cow

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2012/04/04 10:56), David Rientjes wrote:

> On COW, a new hugepage is allocated and charged to the memcg.  If the
> memcg is oom, however, this charge will fail and will return VM_FAULT_OOM
> to the page fault handler 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 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@google.com>
> ---
>  mm/huge_memory.c |    1 +
>  mm/memory.c      |   18 +++++++++++++++---
>  2 files changed, 16 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
> @@ -959,6 +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;



?? how about
==
if (transparent_hugepage_enabled(vma) &&
            !transparent_hugepage_debug_cow())
                new_page = alloc_hugepage_vma(transparent_hugepage_defrag(vma),
                                              vma, haddr, numa_node_id(), 0);
        else
                new_page = NULL;
	
if (unlikely(mem_cgroup_newpage_charge(new_page, mm, GFP_KERNEL))) {
                put_page(new_page);
                new_page = NULL; /* never OOM, just cause fallback */
}

if (unlikely(!new_page)) {
                count_vm_event(THP_FAULT_FALLBACK);
                ret = do_huge_pmd_wp_page_fallback(mm, vma, address,
                                                   pmd, orig_pmd, page, haddr);
                put_page(page);
                goto out;
}
==
?

Thanks,
-Kame

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