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SubjectRe: [PATCH 12/19] mm: memcontrol: convert anon and file-thp to new mem_cgroup_charge() API
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> On May 12, 2020, at 5:58 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Tue, May 12, 2020 at 10:38:54AM -0400, Qian Cai wrote:
>>> On May 8, 2020, at 2:30 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
>>>
>>> With the page->mapping requirement gone from memcg, we can charge anon
>>> and file-thp pages in one single step, right after they're allocated.
>>>
>>> This removes two out of three API calls - especially the tricky commit
>>> step that needed to happen at just the right time between when the
>>> page is "set up" and when it's "published" - somewhat vague and fluid
>>> concepts that varied by page type. All we need is a freshly allocated
>>> page and a memcg context to charge.
>>>
>>> v2: prevent double charges on pre-allocated hugepages in khugepaged
>>>
>>> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
>>> Reviewed-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>>> ---
>>> include/linux/mm.h | 4 +---
>>> kernel/events/uprobes.c | 11 +++--------
>>> mm/filemap.c | 2 +-
>>> mm/huge_memory.c | 9 +++------
>>> mm/khugepaged.c | 35 ++++++++++-------------------------
>>> mm/memory.c | 36 ++++++++++--------------------------
>>> mm/migrate.c | 5 +----
>>> mm/swapfile.c | 6 +-----
>>> mm/userfaultfd.c | 5 +----
>>> 9 files changed, 31 insertions(+), 82 deletions(-)
>> []
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>
>>> @@ -1198,10 +1193,11 @@ static void collapse_huge_page(struct mm_struct *mm,
>>> out_up_write:
>>> up_write(&mm->mmap_sem);
>>> out_nolock:
>>> + if (*hpage)
>>> + mem_cgroup_uncharge(*hpage);
>>> trace_mm_collapse_huge_page(mm, isolated, result);
>>> return;
>>> out:
>>> - mem_cgroup_cancel_charge(new_page, memcg);
>>> goto out_up_write;
>>> }
>> []
>>
>> Some memory pressure will crash this new code. It looks like somewhat racy.
>>
>> if (!page->mem_cgroup)
>>
>> where page == NULL in mem_cgroup_uncharge().
>
> Thanks for the report, sorry about the inconvenience.
>
> Hm, the page is exclusive at this point, nobody else should be
> touching it. After all, khugepaged might reuse the preallocated page
> for another pmd if this one fails to collapse.
>
> Looking at the code, I think it's page itself that's garbage, not
> page->mem_cgroup changing. If you have CONFIG_NUMA and the allocation
> fails, *hpage could contain an ERR_PTR instead of being NULL.
>
> I think we need the following fixlet:

Yes, I have NUMA here.

Stephen, can you pick this up for first before Andrew has a chance to push out the next mmotm hopefully contain this fix?

https://lore.kernel.org/lkml/20200512215813.GA487759@cmpxchg.org/

>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index f2e0a5e5cfbb..f6161e17da26 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1193,7 +1193,7 @@ static void collapse_huge_page(struct mm_struct *mm,
> out_up_write:
> up_write(&mm->mmap_sem);
> out_nolock:
> - if (*hpage)
> + if (!IS_ERR_OR_NULL(*hpage))
> mem_cgroup_uncharge(*hpage);
> trace_mm_collapse_huge_page(mm, isolated, result);
> return;
> @@ -1928,7 +1928,7 @@ static void collapse_file(struct mm_struct *mm,
> unlock_page(new_page);
> out:
> VM_BUG_ON(!list_empty(&pagelist));
> - if (*hpage)
> + if (!IS_ERR_OR_NULL(*hpage))
> mem_cgroup_uncharge(*hpage);
> /* TODO: tracepoints */
> }

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