RFC Critical Page Pool

We have a clustering product that needs to be able to guarantee that the networking system won't stop functioning in the case of OOM/low memory condition. The current mempool system is inadequate because to keep the whole networking stack functioning, we need more than 1 or 2 slab caches to be guaranteed. We need to guarantee that any request made with a specific flag will succeed, assuming of course that you've made your "critical page pool" big enough.

The following patch series implements such a critical page pool. It creates 2 userspace triggers:

/proc/sys/vm/critical_pages: write the number of pages you want to reserve for the critical pool into this file

/proc/sys/vm/in_emergency: write a non-zero value to tell the kernel that the system is in an emergency state and authorize the kernel to dip into the critical pool to satisfy critical allocations.

We mark critical allocations with the __GFP_CRITICAL flag, and when the system is in an emergency state, we are allowed to delve into this pool to satisfy __GFP_CRITICAL allocations that cannot be satisfied through the normal means.

Feedback on our approach would be appreciated.

Thanks!

-Matt

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