linux-kernel - Re: [PATCH v3 00/20] Speculative page faults

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Message-ID: <20171005233206.vpg446q5k2r4g27r@ast-mbp>
Date:   Thu, 5 Oct 2017 16:32:07 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kirill@...temov.name, Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>, dave@...olabs.net,
        Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, haren@...ux.vnet.ibm.com,
        Anshuman Khandual <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" <x86@...nel.org>
Subject: Re: [PATCH v3 00/20] Speculative page faults

On Wed, Oct 04, 2017 at 08:50:49AM +0200, Laurent Dufour wrote:
> On 25/09/2017 18:27, Alexei Starovoitov wrote:
> > On Mon, Sep 18, 2017 at 12:15 AM, Laurent Dufour
> > <ldufour@...ux.vnet.ibm.com> wrote:
> >> Despite the unprovable lockdep warning raised by Sergey, I didn't get any
> >> feedback on this series.
> >>
> >> Is there a chance to get it moved upstream ?
> > 
> > what is the status ?
> > We're eagerly looking forward for this set to land,
> > since we have several use cases for tracing that
> > will build on top of this set as discussed at Plumbers.
> 
> Hi Alexei,
> 
> Based on Plumber's note [1], it sounds that the use case is tied to the BPF
> tracing where a call tp find_vma() call will be made on a process's context
> to fetch user space's symbols.
> 
> Am I right ?
> Is the find_vma() call made in the context of the process owning the mm
> struct ?

Hi Laurent,

we're thinking about several use cases on top of your work.
First one is translation of user address to file_handle where
we need to do find_vma() from preempt_disabled context of bpf program.
My understanding that srcu should solve that nicely.
Second is making probe_read() to try harder when address is causing
minor fault. We're thinking that find_vma() followed by some new
light weight filemap_access() that doesn't sleep will do the trick.
In both cases the program will be accessing current->mm