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Message-ID: <20230821041325.GA2341515@ik1-406-35019.vs.sakura.ne.jp>
Date: Mon, 21 Aug 2023 13:13:25 +0900
From: Naoya Horiguchi <naoya.horiguchi@...ux.dev>
To: Tong Tiangen <tongtiangen@...wei.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Naoya Horiguchi <naoya.horiguchi@....com>,
Miaohe Lin <linmiaohe@...wei.com>, wangkefeng.wang@...wei.com,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memory-failure: use rcu lock instead of
tasklist_lock when collect_procs()
On Mon, Aug 21, 2023 at 10:25:34AM +0800, Tong Tiangen wrote:
> We found a softlock issue in our test, analyzed the logs, and found that
> the relevant CPU call trace as follows:
>
> CPU0:
> _do_fork
> -> copy_process()
> -> write_lock_irq(&tasklist_lock) //Disable irq,waiting for
> //tasklist_lock
>
> CPU1:
> wp_page_copy()
> ->pte_offset_map_lock()
> -> spin_lock(&page->ptl); //Hold page->ptl
> -> ptep_clear_flush()
> -> flush_tlb_others() ...
> -> smp_call_function_many()
> -> arch_send_call_function_ipi_mask()
> -> csd_lock_wait() //Waiting for other CPUs respond
> //IPI
>
> CPU2:
> collect_procs_anon()
> -> read_lock(&tasklist_lock) //Hold tasklist_lock
> ->for_each_process(tsk)
> -> page_mapped_in_vma()
> -> page_vma_mapped_walk()
> -> map_pte()
> ->spin_lock(&page->ptl) //Waiting for page->ptl
>
> We can see that CPU1 waiting for CPU0 respond IPI,CPU0 waiting for CPU2
> unlock tasklist_lock, CPU2 waiting for CPU1 unlock page->ptl. As a result,
> softlockup is triggered.
>
> For collect_procs_anon(), we will not modify the tasklist, but only perform
> read traversal. Therefore, we can use rcu lock instead of spin lock
> tasklist_lock, from this, we can break the softlock chain above.
>
> The same logic can also be applied to:
> - collect_procs_file()
> - collect_procs_fsdax()
> - collect_procs_ksm()
> - find_early_kill_thread()
>
> Signed-off-by: Tong Tiangen <tongtiangen@...wei.com>
Acked-by: Naoya Horiguchi <naoya.horiguchi@....com>
> ---
> Changes since RFC[1]:
> - 1. According to Naoya's suggestion, modify the tasklist_lock in the
> comment about locking order in mm/filemap.c.
> - 2. According to Kefeng's suggestion, optimize the implementation of
> find_early_kill_thread() without functional changes.
> - 3. Modify the title description.
>
> [1] https://lore.kernel.org/lkml/20230815130154.1100779-1-tongtiangen@huawei.com/
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