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Date: Tue, 3 Feb 2015 16:33:03 +0100
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: Michal Hocko <mhocko@...e.cz>
Cc: Linux API <linux-api@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Oleg Nesterov <oleg@...hat.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] fork: report pid reservation failure properly
Hi Michal,
On 3 February 2015 at 16:05, Michal Hocko <mhocko@...e.cz> wrote:
> Hi,
> while debugging an unexpected ENOMEM from fork (there was no memory
> pressure and OVERCOMMIT_ALWAYS) I have found out that fork returns
> ENOMEM even when not short on memory.
>
> In this particular case it was due to depleted pid space which is
> documented to return EAGAIN in man pages.
>
> Here is a quick fix up.
Could you summarize briefly what the user-space visible change is
here? It is not so obvious from your message. I believe you're turning
some cases of ENOMEM into EAGAIN, right? Note, by the way, that if I
understandwhat you intend, this change would bring the implementation
closer to POSIX, which specifies:
EAGAIN The system lacked the necessary resources to create
another process, or the system-imposed limit on the total
number of processes under execution system-wide or by a
single user {CHILD_MAX} would be exceeded.
Thanks,
Michael
> I am sending that as a RFC because I am not
> entirely sure about interaction with pid namespace which might cause
> fork to fail. pid_ns_prepare_proc might return few error codes which are
> not documented by man page but I am not sure whether that is actually
> going to happen on a properly configured system.
>
> There doesn't seem to be a good error code for an already "closed"
> namespace (PIDNS_HASH_ADDING disabled) as well. I've chosen EBUSY but
> that might be completely wrong. I am also wondering how would this
> error case happen in the first place because parent should still exist
> while fork is going on so the pid namespace shouldn't go away but I
> smell this might have something to do with setns or something similar.
>
> Any feedback would be appreciated.
> ---
> From 2a576175345fffa04da19cd51a25b7d94187b5f9 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@...e.cz>
> Date: Fri, 30 Jan 2015 14:50:15 +0100
> Subject: [PATCH] fork: report pid reservation failure properly
>
> copy_process will report any failure in alloc_pid as ENOMEM currently
> which is misleading because the pid allocation might fail not only when
> the memory is short but also when the pid space is consumed already.
>
> The current man page even mentions this case:
> "
> EAGAIN
>
> A system-imposed limit on the number of threads was encountered.
> There are a number of limits that may trigger this error: the
> RLIMIT_NPROC soft resource limit (set via setrlimit(2)), which
> limits the number of processes and threads for a real user ID, was
> reached; the kernel's system-wide limit on the number of processes
> and threads, /proc/sys/kernel/threads-max, was reached (see
> proc(5)); or the maximum number of PIDs, /proc/sys/kernel/pid_max,
> was reached (see proc(5)).
> "
>
> so the current behavior is also incorrect wrt. documentation. This patch
> simply propagates error code from alloc_pid and makes sure we return
> -EAGAIN due to reservation failure.
>
> There is one side effect of the change which might be unexpected.
> alloc_pid might fail even when the repear in the pid namespace is dead
> (the namespace basically disallows all new processes) and there is no
> good error code which would match documented ones. I've taken EBUSY
> because it felt like a best fit for the condition - namespace is busy
> tearing down all the infrastructure.
>
> Signed-off-by: Michal Hocko <mhocko@...e.cz>
> ---
> kernel/fork.c | 5 +++--
> kernel/pid.c | 19 +++++++++++--------
> 2 files changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 3ce8d57cff09..c37b88a162c5 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1405,10 +1405,11 @@ static struct task_struct *copy_process(unsigned long clone_flags,
> goto bad_fork_cleanup_io;
>
> if (pid != &init_struct_pid) {
> - retval = -ENOMEM;
> pid = alloc_pid(p->nsproxy->pid_ns_for_children);
> - if (!pid)
> + if (IS_ERR(pid)) {
> + retval = PTR_ERR(pid);
> goto bad_fork_cleanup_io;
> + }
> }
>
> p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 82430c858d69..c1a5875bd1ab 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -207,7 +207,7 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
> }
> pid = mk_pid(pid_ns, map, offset);
> }
> - return -1;
> + return -EAGAIN;
> }
>
> int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
> @@ -298,17 +298,20 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> int i, nr;
> struct pid_namespace *tmp;
> struct upid *upid;
> + int retval;
>
> pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
> if (!pid)
> - goto out;
> + return ERR_PTR(-ENOMEM);
>
> tmp = ns;
> pid->level = ns->level;
> for (i = ns->level; i >= 0; i--) {
> nr = alloc_pidmap(tmp);
> - if (nr < 0)
> + if (IS_ERR_VALUE(nr)) {
> + retval = nr;
> goto out_free;
> + }
>
> pid->numbers[i].nr = nr;
> pid->numbers[i].ns = tmp;
> @@ -316,7 +319,7 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> }
>
> if (unlikely(is_child_reaper(pid))) {
> - if (pid_ns_prepare_proc(ns))
> + if ((retval = pid_ns_prepare_proc(ns)))
> goto out_free;
> }
>
> @@ -327,8 +330,10 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>
> upid = pid->numbers + ns->level;
> spin_lock_irq(&pidmap_lock);
> - if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
> + if (!(ns->nr_hashed & PIDNS_HASH_ADDING)) {
> + retval = -EBUSY;
> goto out_unlock;
> + }
> for ( ; upid >= pid->numbers; --upid) {
> hlist_add_head_rcu(&upid->pid_chain,
> &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
> @@ -336,7 +341,6 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> }
> spin_unlock_irq(&pidmap_lock);
>
> -out:
> return pid;
>
> out_unlock:
> @@ -348,8 +352,7 @@ out_free:
> free_pidmap(pid->numbers + i);
>
> kmem_cache_free(ns->pid_cachep, pid);
> - pid = NULL;
> - goto out;
> + return ERR_PTR(retval);
> }
>
> void disable_pid_allocation(struct pid_namespace *ns)
> --
> 2.1.4
>
> --
> Michal Hocko
> SUSE Labs
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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