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Date: Fri, 26 Jan 2024 16:00:17 +0900
From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
	Jiexun Wang <wangjiexun@tinylab.org>, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH] mm/madvise: don't forget to leave lazy MMU mode in
 madvise_cold_or_pageout_pte_range()
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References: <20230921174101.8e6271422a857af5414ce0a0@linux-foundation.org>
 <20240126032608.355899-1-senozhatsky@chromium.org>
 <20240125225336.6a444c01d9d9812a23a6890b@linux-foundation.org>
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On (24/01/25 22:53), Andrew Morton wrote:
> Date: Thu, 25 Jan 2024 22:53:36 -0800
> From: Andrew Morton <akpm@linux-foundation.org>
> To: Sergey Senozhatsky <senozhatsky@chromium.org>
> Cc: Jiexun Wang <wangjiexun@tinylab.org>, linux-kernel@vger.kernel.org,
>  linux-mm@kvack.org
> Subject: Re: [PATCH] mm/madvise: don't forget to leave lazy MMU mode in
>  madvise_cold_or_pageout_pte_range()
> Message-Id: <20240125225336.6a444c01d9d9812a23a6890b@linux-foundation.org>
> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu)
> 
> On Fri, 26 Jan 2024 12:25:48 +0900 Sergey Senozhatsky <senozhatsky@chromium.org> wrote:
> 
> > We need to leave lazy MMU mode before unlocking.

It depends on the arch, as far as I understand it.
We can enter lazy MMU mode (on each goto restart) more times than leave
it, and, for isntance, on powerpc that means that we can preempt_disable()
more times than preempt_enable(). That's how enter/leave lazy MMU mode is
implemented there:

static inline void arch_enter_lazy_mmu_mode(void)
{
        struct ppc64_tlb_batch *batch;

        if (radix_enabled())
                return;
        /*
         * apply_to_page_range can call us this preempt enabled when
         * operating on kernel page tables.
         */
        preempt_disable();
        batch = this_cpu_ptr(&ppc64_tlb_batch);
        batch->active = 1;
}

static inline void arch_leave_lazy_mmu_mode(void)
{
        struct ppc64_tlb_batch *batch;

        if (radix_enabled())
                return;
        batch = this_cpu_ptr(&ppc64_tlb_batch);

        if (batch->index)
                __flush_tlb_pending(batch);
        batch->active = 0;
        preempt_enable();
}

> What might be the userspace-visible effects of this?
> 
> > Fixes: b2f557a21bc8 ("mm/madvise: add cond_resched() in madvise_cold_or_pageout_pte_range()"
> > Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> 
> I'll add a cc:stable.

Thanks.