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
