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

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Date:   Mon, 13 Feb 2023 10:22:37 -0600
From:   Mike Christie <michael.christie@...cle.com>
To:     Maurizio Lombardi <mlombard@...hat.com>,
        David Jeffery <djeffery@...hat.com>
Cc:     target-devel@...r.kernel.org,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Laurence Oberman <loberman@...hat.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 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.