linux-kernel - Re: [PATCH] scsi: target: iscsi: set memalloc

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Date:   Mon, 13 Feb 2023 11:25:57 -0500
From:   Laurence Oberman <loberman@...hat.com>
To:     Mike Christie <michael.christie@...cle.com>,
        Maurizio Lombardi <mlombard@...hat.com>,
        David Jeffery <djeffery@...hat.com>
Cc:     target-devel@...r.kernel.org,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scsi: target: iscsi: set memalloc_noio with loopback
 network connections

On Mon, 2023-02-13 at 10:22 -0600, Mike Christie wrote:
> On 2/13/23 5:59 AM, Maurizio Lombardi wrote:
> > st 8. 2. 2023 v 21:10 odesílatel David Jeffery <djeffery@...hat.com
> > > napsal:
> > > 
> > > +       /*
> > > +        * If the iscsi connection is over a loopback device from
> > > using
> > > +        * iscsi and iscsit on the same system, we need to set
> > > memalloc_noio to
> > > +        * prevent memory allocation deadlocks between target and
> > > initiator.
> > > +        */
> > > +       rcu_read_lock();
> > > +       dst = rcu_dereference(conn->sock->sk->sk_dst_cache);
> > > +       if (dst && dst->dev && dst->dev->flags & IFF_LOOPBACK)
> > > +               loopback = true;
> > > +       rcu_read_unlock();
> > 
> > Hi Mike,
> > I tested it, it works. The customer also confirmed that it fixes
> > the
> > deadlock on his setup.
> 
> You never responded about why/how it's used in production. Is it some
> sort
> of clustering or container or what?
> 
> The login related code can still swing back on you if it's run for a
> relogin.
> It would happen if we overqueue and a nop timesout because the iscsi
> recv thread
> is waiting for backend resources like a request/queue slot, or if
> management tools
> disable/enable the tpgt for reconfigs, etc.
> 

Hi Mike,
The use case described is as follows:


"This customer moved their on-premise system to the cloud.
Their on-premise system runs with two servers and one external storage
and uses data mirroring software to mirror data.

When moving to the cloud, customer wanted to implement a data mirror
using data mirror software with two instances to reduce the cost of
using the cloud infrastructure.
To build a system with two instances, we use iSCSI to mirror data
between a local disk on one instance and a local disk on the other
instance.
We coexist iSCSI initiator and target so that data mirroring software
can access each disk through a unified interface."

Thanks
Laurence