linux-kernel - Re: [PATCH 0/4] IB: decrease large contigous allocation

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Date:   Wed, 26 Sep 2018 20:00:12 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Jan Dakinevich <jan.dakinevich@...tuozzo.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Doug Ledford <dledford@...hat.com>,
        Yishai Hadas <yishaih@...lanox.com>,
        Parav Pandit <parav@...lanox.com>,
        Mark Bloch <markb@...lanox.com>,
        Daniel Jurgens <danielj@...lanox.com>,
        Kees Cook <keescook@...omium.org>,
        Kamal Heib <kamalheib1@...il.com>,
        Bart Van Assche <bvanassche@....org>,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        Denis Lunev <den@...tuozzo.com>,
        Konstantin Khorenko <khorenko@...tuozzo.com>
Subject: Re: [PATCH 0/4] IB: decrease large contigous allocation

On Wed, Sep 26, 2018 at 06:43:42PM +0300, Jan Dakinevich wrote:
> On Wed, 19 Sep 2018 00:23:51 +0300
> Leon Romanovsky <leon@...nel.org> wrote:
>
> > On Tue, Sep 18, 2018 at 08:46:23AM -0600, Jason Gunthorpe wrote:
> > > On Tue, Sep 18, 2018 at 04:03:42PM +0300, Jan Dakinevich wrote:
> > > > The size of mlx4_ib_device became too large to be allocated as
> > > > whole contigous block of memory. Currently it takes about 55K. On
> > > > architecture with 4K page it means 3rd order.
> > > >
> > > > This patch series makes an attempt to split mlx4_ib_device into
> > > > several parts and allocate them with less expensive kvzalloc
> > >
> > > Why split it up? Any reason not to just allocate the whole thing
> > > with kvzalloc?
> >
>
> This allocation could be triggered by userspace. It means that at
> _arbitrary_ time kernel could be asked for high order allocation.
>
> This case is considered unacceptable for system under significant load,
> since kernel would try to satisfy this memory request wasting the
> overall performance.

In such case, you won't do very much with mlx4_ib device. It will be
unusable.

>
> > And before we are rushing to dissect mlx4_ib driver, can you
> > explain the rationale behind this change? The mlx4_ib driver
> > represents high-performance device which needs enough memory
> > resources to operate. Those devices are limited by number
> > of PCIs and SRIOV VFs (upto 126) and very rare allocated/deallocated.
> >
> > I would like to see real rationale behind such change.
> >
> > Thanks
> >
> > >
> > > Jason
>
>
>
> --
> Best regards
> Jan Dakinevich

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