Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966168AbbBCPd1 (ORCPT ); Tue, 3 Feb 2015 10:33:27 -0500 Received: from mail-la0-f49.google.com ([209.85.215.49]:60573 "EHLO mail-la0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966017AbbBCPdZ (ORCPT ); Tue, 3 Feb 2015 10:33:25 -0500 MIME-Version: 1.0 Reply-To: mtk.manpages@gmail.com In-Reply-To: <20150203150557.GB8907@dhcp22.suse.cz> References: <20150203150557.GB8907@dhcp22.suse.cz> From: "Michael Kerrisk (man-pages)" Date: Tue, 3 Feb 2015 16:33:03 +0100 Message-ID: Subject: Re: [RFC PATCH] fork: report pid reservation failure properly To: Michal Hocko Cc: Linux API , Andrew Morton , Oleg Nesterov , "Eric W. Biederman" , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7171 Lines: 195 Hi Michal, On 3 February 2015 at 16:05, Michal Hocko 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 > 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 > --- > 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/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/