On Mon 22-05-17 08:00:11, Mikulas Patocka wrote: > > > 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. That would be more than unfortunate and begs for a different solution. The thing is that __GFP_HIGH is not propagated to all allocations in the vmalloc proper. E.g. page table allocations are hardcoded GFP_KERNEL. > > 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. I am not sure I understand. OOM killer is invoked for _all_ allocations <= PAGE_ALLOC_COSTLY_ORDER that do not have __GFP_NORETRY as long as the OOM killer is not disabled (oom_killer_disable) and that only happens from the PM suspend path which makes sure that no userspace is active at the time. AFAIU this is a userspace triggered path and so the later shouldn't apply to it and GFP_KERNEL should be therefore sufficient. Relying to a portion of memory reserves to prevent from deadlock seems fundamentaly broken to me. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
