Re: [PATCH] Add prctl support for controlling PF_MEMALLOC V2

[Mike Christie <mchristi@redhat.com>]

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.
> 
> 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?

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.

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

Will do. Thanks.

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