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: <CAG48ez1xdvsyABAzaBmcmHWwLHzgyb4ORnNE5OG-As5Mq2=MgQ@mail.gmail.com>
Date: Fri, 8 Nov 2024 21:09:17 +0100
From: Jann Horn <jannh@...gle.com>
To: Qi Zheng <zhengqi.arch@...edance.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 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.

> >> +#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...