From: Chengming Zhou <zhouchengming@bytedance.com>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
Andrew Morton <akpm@linux-foundation.org>,
Nhat Pham <nphamcs@gmail.com>, Chris Li <chrisl@kernel.org>,
Huang Ying <ying.huang@intel.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap_swapoff()
Date: Thu, 25 Jan 2024 17:38:21 +0800 [thread overview]
Message-ID: <68b7f87b-7f15-410c-9094-9fe9865b4bc4@bytedance.com> (raw)
In-Reply-To: <CAJD7tkYBLkh82VQ6DmNQNXtnCxTVOY_7JRjARRtDrkKfKyTFSQ@mail.gmail.com>
On 2024/1/25 17:26, Yosry Ahmed wrote:
> On Thu, Jan 25, 2024 at 1:22 AM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
>>
>> On 2024/1/25 17:03, Yosry Ahmed wrote:
>>>>>>>> The second difference is the handling of lru entry, which is easy that we
>>>>>>>> just zswap_lru_del() in tree lock.
>>>>>>>
>>>>>>> Why do we need zswap_lru_del() at all? We should have already isolated
>>>>>>> the entry at that point IIUC.
>>>>>>
>>>>>> I was thinking how to handle the "zswap_lru_putback()" if not writeback,
>>>>>> in which case we can't use the entry actually since we haven't got reference
>>>>>> of it. So we can don't isolate at the entry, and only zswap_lru_del() when
>>>>>> we are going to writeback actually.
>>>>>
>>>>> Why not just call zswap_lru_putback() before we unlock the folio?
>>>>
>>>> When early return because __read_swap_cache_async() return NULL or !folio_was_allocated,
>>>> we don't have a locked folio yet. The entry maybe invalidated and freed concurrently.
>>>
>>> Oh, that path, right.
>>>
>>> If we don't isolate the entry straightaway, concurrent reclaimers will
>>> see the same entry, call __read_swap_cache_async(), find the folio
>>> already in the swapcache and stop shrinking. This is because usually
>>> this means we are racing with swapin and hitting the warmer part of
>>> the zswap LRU.
>>>
>>> I am not sure if this would matter in practice, maybe Nhat knows
>>> better. Perhaps we can rotate the entry in the LRU before calling
>>> __read_swap_cache_async() to minimize the chances of such a race? Or
>>> we can serialize the calls to __read_swap_cache_async() but this may
>>> be an overkill.
>>
>> Also, not sure, rotate the entry maybe good IMHO since we will zswap_lru_del()
>> once we checked the invalidate race.
>
> Not sure what you mean. If we rotate first, we won't have anything to
> do in the failure case (if the folio is not locked). We will have to
> do zswap_lru_del() if actually writeback, yes, but in this case no
> synchronization is needed because the folio is locked, right?
Right, sorry for my confusing expression. We rotate first in lru lock,
and only zswap_lru_del() later if actually writeback.
What I want to mean is that the possibility of seeing the entry on lru list
by another reclaimer is very low, since we rotate and the timing between
__read_swap_cache_async() and zswap_lru_del() should be short.
next prev parent reply other threads:[~2024-01-25 9:38 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-20 2:40 [PATCH 0/2] mm: zswap: simplify zswap_swapoff() Yosry Ahmed
2024-01-20 2:40 ` [PATCH 1/2] mm: swap: update inuse_pages after all cleanups are done Yosry Ahmed
2024-01-22 13:17 ` Chengming Zhou
2024-01-23 8:59 ` Huang, Ying
2024-01-23 9:40 ` Yosry Ahmed
2024-01-23 9:54 ` Yosry Ahmed
2024-01-24 3:13 ` Huang, Ying
2024-01-24 3:20 ` Yosry Ahmed
2024-01-24 3:27 ` Huang, Ying
2024-01-24 4:15 ` Yosry Ahmed
2024-01-20 2:40 ` [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap_swapoff() Yosry Ahmed
2024-01-22 13:13 ` Chengming Zhou
2024-01-22 20:19 ` Johannes Weiner
2024-01-22 20:39 ` Yosry Ahmed
2024-01-23 15:38 ` Johannes Weiner
2024-01-23 15:54 ` Yosry Ahmed
2024-01-23 20:12 ` Johannes Weiner
2024-01-23 21:02 ` Yosry Ahmed
2024-01-24 6:57 ` Yosry Ahmed
2024-01-25 5:28 ` Chris Li
2024-01-25 7:59 ` Yosry Ahmed
2024-01-25 18:55 ` Chris Li
2024-01-25 20:57 ` Yosry Ahmed
2024-01-25 22:31 ` Chris Li
2024-01-25 22:33 ` Yosry Ahmed
2024-01-26 1:09 ` Chris Li
2024-01-24 7:20 ` Chengming Zhou
2024-01-25 5:44 ` Chris Li
2024-01-25 8:01 ` Yosry Ahmed
2024-01-25 19:03 ` Chris Li
2024-01-25 21:01 ` Yosry Ahmed
2024-01-25 7:53 ` Yosry Ahmed
2024-01-25 8:03 ` Yosry Ahmed
2024-01-25 8:30 ` Chengming Zhou
2024-01-25 8:42 ` Yosry Ahmed
2024-01-25 8:52 ` Chengming Zhou
2024-01-25 9:03 ` Yosry Ahmed
2024-01-25 9:22 ` Chengming Zhou
2024-01-25 9:26 ` Yosry Ahmed
2024-01-25 9:38 ` Chengming Zhou [this message]
2024-01-26 0:03 ` Chengming Zhou
2024-01-26 0:05 ` Yosry Ahmed
2024-01-26 0:10 ` Chengming Zhou
2024-01-23 20:30 ` Nhat Pham
2024-01-23 21:04 ` Yosry Ahmed
2024-01-22 21:21 ` Nhat Pham
2024-01-22 22:31 ` Chris Li
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