Re: [PATCH] selftests/core: fix unshare_test with large fs.nr_open

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 14 Aug 2026 18:57:09 +0200 Konstantin Khorenko <khorenko@virtuozzo.com> wrote:

> The test assumes fs.nr_open is close to the default 1048576, but some
> systems set it much higher (e.g. 1073741816). This is systemd's doing:
> since systemd v240 (2018), PID 1 bumps fs.nr_open and fs.file-max to
> their largest possible values on boot, as file descriptors are already
> accounted for by memcg [1].
> 
> In that case, dup2() to nr_open + 64 requires the kernel to allocate a
> file descriptor table with ~1 billion entries, which fails with ENOMEM.

2018.  Can you suggest why we (or I) haven't heard about this?

> Cap the nr_open value used for the test's own arithmetic to a known
> reasonable base value (1048576) and restore the true original value
> once the test has completed.
> 
> [1] https://github.com/systemd/systemd/commit/a8b627aaed409a15260c25988970c795bf963812
>     ("main: bump fs.nr_open + fs.max-file to their largest possible values")

Well, we do want selftests to run well on the kernel with which they
are shipped, so I'm thinking we should backport this into -stable
kernels.  We can probably skip the Fixes:, but a cc:stable should be
added to capture this.

Thoughts?