Re: [PATCH 17/19] dm: get rid of superfluous gfp flags

[Michal Hocko <mhocko@kernel.org>]

[Adding dm-devel@redhat.com to CC]

On Tue 26-04-16 13:20:04, Mikulas Patocka wrote:
> On Fri, 22 Apr 2016, Michal Hocko wrote:
[...]
> > copy_params seems to be called only from the ioctl context which doesn't
> > hold any locks which would lockup during the direct reclaim AFAICS. The
> > git log shows that the code has used PF_MEMALLOC before which is even
> > bigger mystery to me. Could you please clarify why this is GFP_NOIO
> > restricted context? Maybe it needed to be in the past but I do not see
> > any reason for it to be now so unless I am missing something the
> > GFP_KERNEL should be perfectly OK. Also note that GFP_NOIO wouldn't work
> > properly because there are copy_from_user calls in the same path which
> > could page fault and do GFP_KERNEL allocations anyway. I can send follow
> > up cleanups unless I am missing something subtle here.
> 
> The LVM tool calls suspend and resume ioctls on device mapper block 
> devices.
>
> When a device is suspended, any bio sent to the device is held. If the 
> resume ioctl did GFP_KERNEL allocation, the allocation could get stuck 
> trying to write some dirty cached pages to the suspended device.
> 
> The LVM tool and the dmeventd daemon use mlock to lock its address space, 
> so the copy_from_user/copy_to_user call cannot trigger a page fault.

OK, I see, thanks for the clarification! This sounds fragile to me
though. Wouldn't it be better to use the memalloc_noio_save for the
whole copy_params instead? That would force all possible allocations to
not trigger any IO. Something like the following.
---