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

From: Michal Hocko
Date: Mon May 22 2017 - 05:37:44 EST


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? Why
is OOM killer insufficient to allow the further progress?
--
Michal Hocko
SUSE Labs