linux-kernel - Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio

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Message-ID: <7df7fdcb-2c3e-43da-824e-586a348ae840@arm.com>
Date: Tue, 19 Mar 2024 14:40:12 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Yin Fengwei <fengwei.yin@...el.com>, David Hildenbrand
 <david@...hat.com>, "Huang, Ying" <ying.huang@...el.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Gao Xiang <xiang@...nel.org>,
 Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
 Michal Hocko <mhocko@...e.com>, Kefeng Wang <wangkefeng.wang@...wei.com>,
 Barry Song <21cnbao@...il.com>, Chris Li <chrisl@...nel.org>
Subject: Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

On 19/03/2024 02:20, Yin Fengwei wrote:
> 
> 
> On 3/18/24 23:35, Ryan Roberts wrote:
>> On 18/03/2024 10:05, David Hildenbrand wrote:
>>> On 18.03.24 11:00, Yin, Fengwei wrote:
>>>>
>>>>
>>>> On 3/18/2024 10:16 AM, Huang, Ying wrote:
>>>>> Ryan Roberts <ryan.roberts@....com> writes:
>>>>>
>>>>>> Hi Yin Fengwei,
>>>>>>
>>>>>> On 15/03/2024 11:12, David Hildenbrand wrote:
>>>>>>> On 15.03.24 11:49, Ryan Roberts wrote:
>>>>>>>> On 15/03/2024 10:43, David Hildenbrand wrote:
>>>>>>>>> On 11.03.24 16:00, Ryan Roberts wrote:
>>>>>>>>>> Now that swap supports storing all mTHP sizes, avoid splitting large
>>>>>>>>>> folios before swap-out. This benefits performance of the swap-out path
>>>>>>>>>> by eliding split_folio_to_list(), which is expensive, and also sets us
>>>>>>>>>> up for swapping in large folios in a future series.
>>>>>>>>>>
>>>>>>>>>> If the folio is partially mapped, we continue to split it since we want
>>>>>>>>>> to avoid the extra IO overhead and storage of writing out pages
>>>>>>>>>> uneccessarily.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>>>>>>>>>> ---
>>>>>>>>>>      mm/vmscan.c | 9 +++++----
>>>>>>>>>>      1 file changed, 5 insertions(+), 4 deletions(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>>>>>>>> index cf7d4cf47f1a..0ebec99e04c6 100644
>>>>>>>>>> --- a/mm/vmscan.c
>>>>>>>>>> +++ b/mm/vmscan.c
>>>>>>>>>> @@ -1222,11 +1222,12 @@ static unsigned int shrink_folio_list(struct
>>>>>>>>>> list_head
>>>>>>>>>> *folio_list,
>>>>>>>>>>                          if (!can_split_folio(folio, NULL))
>>>>>>>>>>                              goto activate_locked;
>>>>>>>>>>                          /*
>>>>>>>>>> -                     * Split folios without a PMD map right
>>>>>>>>>> -                     * away. Chances are some or all of the
>>>>>>>>>> -                     * tail pages can be freed without IO.
>>>>>>>>>> +                     * Split partially mapped folios map
>>>>>>>>>> +                     * right away. Chances are some or all
>>>>>>>>>> +                     * of the tail pages can be freed
>>>>>>>>>> +                     * without IO.
>>>>>>>>>>                           */
>>>>>>>>>> -                    if (!folio_entire_mapcount(folio) &&
>>>>>>>>>> +                    if (!list_empty(&folio->_deferred_list) &&
>>>>>>>>>>                              split_folio_to_list(folio,
>>>>>>>>>>                                      folio_list))
>>>>>>>>>>                              goto activate_locked;
>>>>>>>>>
>>>>>>>>> Not sure if we might have to annotate that with data_race().
>>>>>>>>
>>>>>>>> I asked that exact question to Matthew in another context bt didn't get a
>>>>>>>> response. There are examples of checking if the deferred list is empty
>>>>>>>> with and
>>>>>>>> without data_race() in the code base. But list_empty() is implemented like
>>>>>>>> this:
>>>>>>>>
>>>>>>>> static inline int list_empty(const struct list_head *head)
>>>>>>>> {
>>>>>>>>       return READ_ONCE(head->next) == head;
>>>>>>>> }
>>>>>>>>
>>>>>>>> So I assumed the READ_ONCE() makes everything safe without a lock? Perhaps
>>>>>>>> not
>>>>>>>> sufficient for KCSAN?
>>>> I don't think READ_ONCE() can replace the lock.
>>
>> But it doesn't ensure we get a consistent value and that the compiler orders the
>> load correctly. There are lots of patterns in the kernel that use READ_ONCE()
>> without a lock and they don't use data_race() - e.g. ptep_get_lockless().
> They (ptep_get_lockless() and deferred_list) have different access pattern
> (or race pattern) here. I don't think they are comparable.
> 
>>
>> It sounds like none of us really understand what data_race() is for, so I guess
>> I'll just do a KCSAN build and invoke the code path to see if it complains.
> READ_ONCE() in list_empty will shutdown the KCSAN also.

OK, I found some time to run the test with KCSAN; nothing fires.

But then I read the docs and looked at the code a bit.
Documentation/dev-tools/kcsan.rst states:

    In an execution, two memory accesses form a *data race* if they *conflict*,
    they happen concurrently in different threads, and at least one of them is a
    *plain access*; they *conflict* if both access the same memory location, and
    at least one is a write.

It also clarifies the READ_ONCE() is a "marked access". So we would have a data
race if there was a concurrent, *plain* write to folio->_deferred_list.next.
This can occur in a couple of places I believe, for example:

deferred_split_folio()
  list_add_tail()
    __list_add()
      new->next = next;

deferred_split_scan()
  list_move()
    list_add()
      __list_add()
        new->next = next;

So if either partially deferred_split_folio() or deferred_split_scan() can run
concurrently with shrink_folio_list(), for the same folio (I beleive both can
can), then we have a race, and this list_empty() check needs to be protected
with data_race(). The race is safe/by design, but it does need to be marked.

I'll fix this in my next version.

Thanks,
Ryan


> 
>>
>>
>>>>
>>>>>>>
>>>>>>> Yeah, there is only one use of data_race with that list.
>>>>>>>
>>>>>>> It was added in f3ebdf042df4 ("THP: avoid lock when check whether THP is in
>>>>>>> deferred list").
>>>>>>>
>>>>>>> Looks like that was added right in v1 of that change [1], so my best guess is
>>>>>>> that it is not actually required.
>>>>>>>
>>>>>>> If not required, likely we should just cleanup the single user.
>>>>>>>
>>>>>>> [1]
>>>>>>> https://lore.kernel.org/linux-mm/20230417075643.3287513-2-fengwei.yin@intel.com/
>>>>>>
>>>>>> Do you have any recollection of why you added the data_race() markup?
>>>>>
>>>>> Per my understanding, this is used to mark that the code accesses
>>>>> folio->_deferred_list without lock intentionally, while
>>>>> folio->_deferred_list may be changed in parallel.  IIUC, this is what
>>>>> data_race() is used for.  Or, my understanding is wrong?
>>>> Yes. This is my understanding also.
>>>
>>> Why don't we have a data_race() in deferred_split_folio() then, before taking
>>> the lock?
>>>
>>> It's used a bit inconsistently here.
>>>
>>