Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params()

[Mikulas Patocka]

On Mon, 22 May 2017, Michal Hocko wrote:

> On Fri 19-05-17 19:43:23, Mikulas Patocka wrote:
> > 
> > 
> > On Fri, 19 May 2017, Michal Hocko wrote:
> > 
> > > On Thu 18-05-17 19:50:46, Junaid Shahid wrote:
> > > > (Adding back the correct linux-mm email address and also adding linux-kernel.)
> > > > 
> > > > On Thursday, May 18, 2017 01:41:33 PM David Rientjes wrote:
> > > [...]
> > > > > Let's ask Mikulas, who changed this from PF_MEMALLOC to __GFP_HIGH, 
> > > > > assuming there was a reason to do it in the first place in two different 
> > > > > ways.
> > > 
> > > Hmm, the old PF_MEMALLOC used to have the following comment
> > >         /*
> > >          * Trying to avoid low memory issues when a device is
> > >          * suspended. 
> > >          */
> > > 
> > > I am not really sure what that means but __GFP_HIGH certainly have a
> > > different semantic than PF_MEMALLOC. The later grants the full access to
> > > the memory reserves while the prior on partial access. If this is _really_
> > > needed then it deserves a comment explaining why.
> > > -- 
> > > Michal Hocko
> > > SUSE Labs
> > 
> > Sometimes, I/O to a device mapper device is blocked until the userspace 
> > daemon dmeventd does some action (for example, when dm-mirror leg fails, 
> > dmeventd needs to mark the leg as failed in the lvm metadata and then 
> > reload the device).
> > 
> > The dmeventd daemon mlocks itself in memory so that it doesn't generate 
> > any I/O. But it must be able to call ioctls. __GFP_HIGH is there so that 
> > the ioctls issued by dmeventd have higher chance of succeeding if some I/O 
> > is blocked, waiting for dmeventd action. It reduces the possibility of 
> > low-memory-deadlock, though it doesn't eliminate it entirely.
> 
> So what happens if the memory reserves are depleted. Do we deadlock?

Yes, it will deadlock.

> Why is OOM killer insufficient to allow the further progress?

I don't know if the OOM killer will or won't be triggered in this 
situation, it depends on the people who wrote the OOM killer.

> -- 
> Michal Hocko
> SUSE Labs

Mikulas