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    SubjectRe: [PATCH] fork: report pid exhaustion correctly
    On Mon,  3 Sep 2018 04:10:16 -0700 ktsanaktsidis@zendesk.com wrote:

    > From: KJ Tsanaktsidis <ktsanaktsidis@zendesk.com>
    >
    > Make the clone and fork syscalls return EAGAIN when the limit on the
    > number of pids /proc/sys/kernel/pid_max is exceeded.
    >
    > Currently, when the pid_max limit is exceeded, the kernel will return
    > ENOSPC from the fork and clone syscalls. This is contrary to the
    > documented behaviour, which explicitly calls out the pid_max case as one
    > where EAGAIN should be returned. It also leads to really confusing error
    > messages in userspace programs which will complain about a lack of disk
    > space when they fail to create processes/threads for this reason.
    >
    > This error is being returned because the alloc_pid function uses the idr
    > api to find a new pid; when there are none available, idr_alloc_cyclic
    > is returns -ENOSPC, and this is being propagated back into userspace.
    >
    > This behaviour has been broken before, and was explicitly fixed in
    > commit 35f71bc0a09a ("fork: report pid reservation failure properly"),
    > so I think -EAGAIN is definitely the right thing to return in this case.
    > The current behaviour change dates from commit 95846ecf9dac ("pid:
    > replace pid bitmap implementation with IDR AIP") and was I believe
    > unintentional.
    >
    > This patch has no impact on the case where allocating a pid fails
    > because the child reaper for the namespace is dead; that case will still
    > return -ENOMEM.

    Thanks. First ever kernel patch? It was a damn good one!

    > Fixes: 95846ecf9dac ("pid: replace pid bitmap implementation with IDR AIP")
    > Signed-off-by: KJ Tsanaktsidis <ktsanaktsidis@zendesk.com>

    I'll add cc:stable here so the fix gets backported into earlier kernels

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