Re: [PATCH v4 0/4] Deterministic charging of shared memory

From: Matthew Wilcox
Date: Sat Nov 20 2021 - 00:02:08 EST

On Fri, Nov 19, 2021 at 08:50:06PM -0800, Mina Almasry wrote:
> 1. One complication to address is the behavior when the target memcg
> hits its memory.max limit because of remote charging. In this case the
> oom-killer will be invoked, but the oom-killer may not find anything
> to kill in the target memcg being charged. Thera are a number of considerations
> in this case:
> 1. It's not great to kill the allocating process since the allocating process
> is not running in the memcg under oom, and killing it will not free memory
> in the memcg under oom.
> 2. Pagefaults may hit the memcg limit, and we need to handle the pagefault
> somehow. If not, the process will forever loop the pagefault in the upstream
> kernel.
> In this case, I propose simply failing the remote charge and returning an ENOSPC
> to the caller. This will cause will cause the process executing the remote
> charge to get an ENOSPC in non-pagefault paths, and get a SIGBUS on the pagefault
> path. This will be documented behavior of remote charging, and this feature is
> opt-in. Users can:
> - Not opt-into the feature if they want.
> - Opt-into the feature and accept the risk of received ENOSPC or SIGBUS and
> abort if they desire.
> - Gracefully handle any resulting ENOSPC or SIGBUS errors and continue their
> operation without executing the remote charge if possible.

Why is ENOSPC the right error instead of ENOMEM?