Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 22 Oct 2019 09:52:34 +1100
From:   Dave Chinner <david@fromorbit.com>
To:     Mike Christie <mchristi@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-scsi@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-block@vger.kernel.org, martin@urbackup.org,
        Damien.LeMoal@wdc.com
Subject: Re: [PATCH] Add prctl support for controlling PF_MEMALLOC V2
Message-ID: <20191021225234.GC2642@dread.disaster.area>
References: <20191021214137.8172-1-mchristi@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20191021214137.8172-1-mchristi@redhat.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Oct 21, 2019 at 04:41:37PM -0500, Mike Christie wrote:
> There are several storage drivers like dm-multipath, iscsi, tcmu-runner,
> amd nbd that have userspace components that can run in the IO path. For
> example, iscsi and nbd's userspace deamons may need to recreate a socket
> and/or send IO on it, and dm-multipath's daemon multipathd may need to
> send IO to figure out the state of paths and re-set them up.
> 
> In the kernel these drivers have access to GFP_NOIO/GFP_NOFS and the
> memalloc_*_save/restore functions to control the allocation behavior,
> but for userspace we would end up hitting a allocation that ended up
> writing data back to the same device we are trying to allocate for.

I think this needs to describe the symptoms this results in. i.e.
that this can result in deadlocking the IO path.

> This patch allows the userspace deamon to set the PF_MEMALLOC* flags
> with prctl during their initialization so later allocations cannot
> calling back into them.
> 
> Signed-off-by: Mike Christie <mchristi@redhat.com>
> ---

....
> +	case PR_SET_MEMALLOC:
> +		if (!capable(CAP_SYS_ADMIN))
> +			return -EPERM;

Wouldn't CAP_SYS_RAWIO (because it's required by kernel IO path
drivers) or CAP_SYS_RESOURCE (controlling memory allocation
behaviour) be more appropriate here?

Which-ever is selected, the use should be added to the list above
the definition of the capability in include/linux/capability.h...

Otherwise looks fine to me.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com