4fe348cfdf
As explained in https://lore.kernel.org/all/20250419183545.1982187-1-shakeel.butt@linux.dev/, writing to memory.max or memory.high triggers synchronous memory reclaim if the limit is lowered. This can end up taking nonnegligible amounts of time, completely blocking pid1 from doing any other work while the reclaim is ongoing. To address this problem, the kernel going to add O_NONBLOCK semantics to memory.max and memory.high. If the file is opened with O_NONBLOCK, the synchronous memory reclaim is skipped and only triggered later without blocking the process writing the file. Let's make sure we make use of this by opening cgroupv2 attribute files with O_NONBLOCK. We opt to do this for all cgroupv2 attribute files, to make sure that if the same problem happens elsewhere in the future and is fixed in the same way, we immediately take advantage of that fix without having to make changes in systemd as well. We probably never want to block when writing cgroupv2 attributes and any cases where we do want to block should indicate so explicitly instead of blocking by default.
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