Re: [PATCH 12/19] mm: memcontrol: convert anon and file-thp to new mem_cgroup_charge() API

[Qian Cai]

> On May 12, 2020, at 5:58 PM, Johannes Weiner <hannes@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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@xxxxxxxxxxx>
>>> Reviewed-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>>> ---
>>> 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@xxxxxxxxxxx/

> 
> 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 */
> }