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

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Date: Fri, 15 Mar 2024 12:12:38 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Yin Fengwei <fengwei.yin@...el.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Huang Ying <ying.huang@...el.com>,
 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 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?

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/

-- 
Cheers,

David / dhildenb