linux-kernel - Re: [PATCH v2 6/7] x86: mm: free page table pages by RCU instead of semi RCU

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Message-ID: <0a27f4c7-fb1a-4fb9-b563-813e0d26fc55@bytedance.com>
Date: Sat, 9 Nov 2024 11:14:21 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Jann Horn <jannh@...gle.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>,
 Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 david@...hat.com, hughd@...gle.com, willy@...radead.org, mgorman@...e.de,
 muchun.song@...ux.dev, vbabka@...nel.org, akpm@...ux-foundation.org,
 zokeefe@...gle.com, rientjes@...gle.com, peterx@...hat.com,
 catalin.marinas@....com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 x86@...nel.org
Subject: Re: [PATCH v2 6/7] x86: mm: free page table pages by RCU instead of
 semi RCU



On 2024/11/9 04:09, Jann Horn wrote:
> On Fri, Nov 8, 2024 at 8:38 AM Qi Zheng <zhengqi.arch@...edance.com> wrote:
>> On 2024/11/8 06:39, Jann Horn wrote:
>>> On Thu, Oct 31, 2024 at 9:14 AM Qi Zheng <zhengqi.arch@...edance.com> wrote:
>>> free_pte
>>>     pte_free_tlb
>>>       __pte_free_tlb
>>>         ___pte_free_tlb
>>>           paravirt_tlb_remove_table
>>>             tlb_remove_table [!CONFIG_PARAVIRT, Xen PV, Hyper-V, KVM]
>>>               [no-free-memory slowpath:]
>>>                 tlb_table_invalidate
>>>                 tlb_remove_table_one
>>>                   tlb_remove_table_sync_one [does IPI for GUP-fast]
>>
>>                     ^
>>                     It seems that this step can be ommitted when
>>                     CONFIG_PT_RECLAIM is enabled, because GUP-fast will
>>                     disable IRQ, which can also serve as the RCU critical
>>                     section.
> 
> Yeah, I think so too.

Will remove this step in the next version.

> 
>>>> +#ifdef CONFIG_PT_RECLAIM
>>>> +static inline void __tlb_remove_table_one_rcu(struct rcu_head *head)
>>>> +{
>>>> +       struct page *page;
>>>> +
>>>> +       page = container_of(head, struct page, rcu_head);
>>>> +       free_page_and_swap_cache(page);
>>>> +}
>>>
>>> Why free_page_and_swap_cache()? Page tables shouldn't have swap cache,
>>> so I think something like put_page() would do the job.
>>
>> Ah, I just did the same thing as __tlb_remove_table(). But I also
>> have the same doubt as you, why does __tlb_remove_table() need to
>> call free_page_and_swap_cache() instead of put_page().
> 
> I think commit 9e52fc2b50de3a1c08b44f94c610fbe998c0031a probably just
> copy-pasted it from a more generic page freeing path...

I guess so. Will use put_page() instead of free_page_and_swap_cache()
in the next version. But for __tlb_remove_table(), I prefer to send
a separate patch to modify.

Thanks!