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

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Date:   Tue, 23 Apr 2019 11:27:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Laurent Dufour <ldufour@...ux.ibm.com>
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>,
        kemi.wang@...el.com, Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Ganesh Mahendran <opensource.ganesh@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Punit Agrawal <punitagrawal@...il.com>,
        vinayak menon <vinayakm.list@...il.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        zhong jiang <zhongjiang@...wei.com>,
        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,
        haren@...ux.vnet.ibm.com, npiggin@...il.com,
        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()

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.

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.

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.

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