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Subject: Re: [PATCH] Add prctl support for controlling PF_MEMALLOC V2
To:     Dave Chinner <david@fromorbit.com>
References: <20191021214137.8172-1-mchristi@redhat.com>
 <20191021225234.GC2642@dread.disaster.area>
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
From:   Mike Christie <mchristi@redhat.com>
Message-ID: <5DAF2363.2070204@redhat.com>
Date:   Tue, 22 Oct 2019 10:42:27 -0500
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On 10/21/2019 05:52 PM, Dave Chinner wrote:
> 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.
>=20
> I think this needs to describe the symptoms this results in. i.e.
> that this can result in deadlocking the IO path.
>=20
>> 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>
>> ---
>=20
> ....
>> +=09case PR_SET_MEMALLOC:
>> +=09=09if (!capable(CAP_SYS_ADMIN))
>> +=09=09=09return -EPERM;
>=20
> 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?

I think I misread a review comment last posting. I will use
CAP_SYS_RESROUCE on the next resend if people do not have any objections.

>=20
> Which-ever is selected, the use should be added to the list above
> the definition of the capability in include/linux/capability.h...
>=20

Will do. Thanks.

> Otherwise looks fine to me.
>=20
> Cheers,
>=20
> Dave.
>=20