Michal Hocko wrote:
> Another issue is that GFP_NOFS is quite often used without any obvious
> reason. It is not clear which lock is held and could be taken from
> the reclaim path. Wouldn't it be much better if the no-recursion
> behavior was bound to the lock scope rather than particular allocation
> request? We already have something like this for PM
> pm_res{trict,tore}_gfp_mask resp. memalloc_noio_{save,restore}. It
> would be great if we could unify this and use the context based NOFS
> in the FS.
Yes, I do want it. I think some of LSM hooks are called from GFP_NOFS context
but it is too difficult for me to tell whether we are using GFP_NOFS correctly.
> First we shouldn't retry endlessly and rather fail the allocation and
> allow the FS to handle the error. As per my experiments most FS cope
> with that quite reasonably. Btrfs unfortunately handles many of those
> failures by BUG_ON which is really unfortunate.
If it turned out that we are using GFP_NOFS from LSM hooks correctly,
I'd expect such GFP_NOFS allocations retry unless SIGKILL is pending.
Filesystems might be able to handle GFP_NOFS allocation failures. But
userspace might not be able to handle system call failures caused by
GFP_NOFS allocation failures; OOM-unkillable processes might unexpectedly
terminate as if they are OOM-killed. Would you please add GFP_KILLABLE
to list of the topics?
> - OOM killer has been discussed a lot throughout this year. We have
> discussed this topic the last year at LSF and there has been quite some
> progress since then. We have async memory tear down for the OOM victim
> [2] which should help in many corner cases. We are still waiting
> to make mmap_sem for write killable which would help in some other
> classes of corner cases. Whatever we do, however, will not work in
> 100% cases. So the primary question is how far are we willing to go to
> support different corner cases. Do we want to have a
> panic_after_timeout global knob, allow multiple OOM victims after
> a timeout?
A sequence for handling any corner case (as long as OOM killer is
invoked) was proposal at
http://lkml.kernel.org/r/201601222259.GJB90663.MLOJtFFOQFVHSO@xxxxxxxxxxxxxxxxxxx .
> - sysrq+f to trigger the oom killer follows some heuristics used by the
> OOM killer invoked by the system which means that it is unreliable
> and it might skip to kill any task without any explanation why. The
> semantic of the knob doesn't seem to clear and it has been even
> suggested [3] to remove it altogether as an unuseful debugging aid. Is
> this really a general consensus?
Even if we remove SysRq-f from future kernels, please give us a fix for
current kernels. ;-)
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