linux-kernel - Re: [PATCH v12 21/31] mm: Introduce find_vma

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Message-Id: <ea141f88-a0d3-df97-5141-706fa18b7ad3@linux.ibm.com>
Date:   Wed, 24 Apr 2019 09:57:05 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     akpm@...ux-foundation.org, mhocko@...nel.org, kirill@...temov.name,
        ak@...ux.intel.com, dave@...olabs.net, jack@...e.cz,
        Matthew Wilcox <willy@...radead.org>,
        aneesh.kumar@...ux.ibm.com, 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>,
        sergey.senozhatsky.work@...il.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
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        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Ganesh Mahendran <opensource.ganesh@...il.com>,
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        Punit Agrawal <punitagrawal@...il.com>,
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        Haiyan Song <haiyanx.song@...el.com>,
        Balbir Singh <bsingharora@...il.com>, sj38.park@...il.com,
        Michel Lespinasse <walken@...gle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
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        paulmck@...ux.vnet.ibm.com, Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v12 21/31] mm: Introduce find_vma_rcu()

Le 23/04/2019 à 11:27, Peter Zijlstra a écrit :
> On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote:
>> This allows to search for a VMA structure without holding the mmap_sem.
>>
>> The search is repeated while the mm seqlock is changing and until we found
>> a valid VMA.
>>
>> While under the RCU protection, a reference is taken on the VMA, so the
>> caller must call put_vma() once it not more need the VMA structure.
>>
>> At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a
>> reference is taken to the VMA by calling get_vma().
>>
>> When removing a VMA from the MM RB tree, the VMA is not release immediately
>> but at the end of the RCU grace period through vm_rcu_put(). This ensures
>> that the VMA remains allocated until the end the RCU grace period.
>>
>> Since the vm_file pointer, if valid, is released in put_vma(), there is no
>> guarantee that the file pointer will be valid on the returned VMA.
> 
> What I'm missing here, and in the previous patch introducing the
> refcount (also see refcount_t), is _why_ we need the refcount thing at
> all.

The need for the VMA's refcount is to ensure that the VMA will remain 
until the end of the SPF handler. This is a consequence of the use of 
RCU instead of SRCU to protect the RB tree.

I was not aware of the refcount_t type, it would be better here to avoid 
wrapping.

> My original plan was to use SRCU, which at the time was not complete
> enough so I abused/hacked preemptible RCU, but that is no longer the
> case, SRCU has all the required bits and pieces.

When I did test using SRCU it was involving a lot a scheduling to run 
the SRCU callback mechanism. In some workload the impact on the 
perfomance was significant [1].

I can't see this overhead using RCU.

> 
> Also; the initial motivation was prefaulting large VMAs and the
> contention on mmap was killing things; but similarly, the contention on
> the refcount (I did try that) killed things just the same.

Doing prefaulting should be doable, I'll try to think further about that.

Regarding the refcount, I should I missed something, this is an atomic 
counter, so there should not be contention on it but cache exclusivity, 
not ideal I agree but I can't see what else to use here.

> So I'm really sad to see the refcount return; and without any apparent
> justification.

I'm not opposed to use another mechanism here, but SRCU didn't show good 
performance with some workload, and I can't see how to use RCU without a 
reference counter here. So please, advise.

Thanks,
Laurent.

[1] 
https://lore.kernel.org/linux-mm/7ca80231-fe02-a3a7-84bc-ce81690ea051@intel.com/