linux-kernel - Re: [PATCH v6 16/24] mm: Protect mm

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Date:   Mon, 15 Jan 2018 18:42:11 +0100
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
        akpm@...ux-foundation.org, kirill@...temov.name,
        ak@...ux.intel.com, mhocko@...nel.org, dave@...olabs.net,
        jack@...e.cz, benh@...nel.crashing.org, mpe@...erman.id.au,
        paulus@...ba.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, sergey.senozhatsky.work@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v6 16/24] mm: Protect mm_rb tree with a rwlock

Hi Matthew,

Thanks for reviewing this series.

On 12/01/2018 19:48, Matthew Wilcox wrote:
> On Fri, Jan 12, 2018 at 06:26:00PM +0100, Laurent Dufour wrote:
>> -static void __vma_rb_erase(struct vm_area_struct *vma, struct rb_root *root)
>> +static void __vma_rb_erase(struct vm_area_struct *vma, struct mm_struct *mm)
>>  {
>> +	struct rb_root *root = &mm->mm_rb;
>>  	/*
>>  	 * Note rb_erase_augmented is a fairly large inline function,
>>  	 * so make sure we instantiate it only once with our desired
>>  	 * augmented rbtree callbacks.
>>  	 */
>> +#ifdef CONFIG_SPF
>> +	write_lock(&mm->mm_rb_lock);
>> +#endif
>>  	rb_erase_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
>> +#ifdef CONFIG_SPF
>> +	write_unlock(&mm->mm_rb_lock); /* wmb */
>> +#endif
> 
> I can't say I love this.  Have you considered:
> 
> #ifdef CONFIG_SPF
> #define vma_rb_write_lock(mm)	write_lock(&mm->mm_rb_lock)
> #define vma_rb_write_unlock(mm)	write_unlock(&mm->mm_rb_lock)
> #else
> #define vma_rb_write_lock(mm)	do { } while (0)
> #define vma_rb_write_unlock(mm)	do { } while (0)
> #endif

I haven't consider this, but this sounds to be smarter. I'll do that.

> Also, SPF is kind of uninformative.  CONFIG_MM_SPF might be better?
> Or perhaps even CONFIG_SPECULATIVE_PAGE_FAULT, just to make it really
> painful to do these one-liner ifdefs that make the code so hard to read.

Thomas also complained about that, and I agree, SPF is quite cryptic. This
being said, I don't think that CONFIG_MM_SPF will be far better, so I'll
change this define to CONFIG_SPECULATIVE_PAGE_FAULT, even if it's longer,
it should not be too much present in the code.

Thanks,
Laurent.