linux-kernel - Re: [v6,1/2] pid: Replace pid bitmap implementation with IDR API

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Date:   Fri, 20 Oct 2017 10:21:01 -0700
From:   Andrei Vagin <avagin@...tuozzo.com>
To:     Gargi Sharma <gs051095@...il.com>
Cc:     Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
        Rik van Riel <riel@...riel.com>,
        Julia Lawall <julia.lawall@...6.fr>,
        Andrew Morton <akpm@...ux-foundation.org>, mingo@...nel.org,
        pasha.tatashin@...cle.com, ktkhai@...tuozzo.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Christoph Hellwig <hch@...radead.org>, lkp@...el.com,
        tony.luck@...el.com
Subject: Re: [v6,1/2] pid: Replace pid bitmap implementation with IDR API

On Fri, Oct 20, 2017 at 05:06:47PM +0100, Gargi Sharma wrote:
> On Thu, Oct 19, 2017 at 5:18 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> > On 10/19, Andrei Vagin wrote:
> >>
> >> Hi Gargi,
> >>
> >> This patch breaks CRIU, because it changes a meaning of ns_last_pid.
> >
> > ...
> >
> >> > @@ -311,7 +297,7 @@ static int pid_ns_ctl_handler(struct ctl_table *table, int write,
> >> >      * it should synchronize its usage with external means.
> >> >      */
> >> >
> >> > -   tmp.data = &pid_ns->last_pid;
> >> > +   tmp.data = &pid_ns->idr.idr_next;
> >
> > Ah, yes, off-by-one error...
> >
> > Gargi, I don't think you need to make another version, I'd suggest you to send
> > the trivial fix to Andrew, afaics you just need to replace these 2 lines with
> >
> >         unsigned int last;
> >         int err;
> >
> >         tmp.data = &last;
> >         err = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
> >         if (!err)
> >                 idr_set_cursor(&pid_ns->idr, last + 1);
> >         return err;
> I'm not sure entirely understand how this takes care of rolling over of PIDs?
> Can we ignore that? If yes, won't the tests for CRIU still break?

Gargi, I don't understand what you mean. Could you elaborate? Do you
mean a case when idr_next is bigger than pid_max? I think this logic
remains the same what we had before switching to idr.

CRIU tests works with a following patch. It is slightly modified version
of Oleg's patch.

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index fea2c24..1c791b3 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -287,6 +287,7 @@ static int pid_ns_ctl_handler(struct ctl_table
*table, int write,
 {
        struct pid_namespace *pid_ns = task_active_pid_ns(current);
        struct ctl_table tmp = *table;
+       int ret;
 
        if (write && !ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
                return -EPERM;
@@ -298,7 +299,12 @@ static int pid_ns_ctl_handler(struct ctl_table
*table, int write,
         */
 
        tmp.data = &pid_ns->idr.idr_next;
-       return proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
+       ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
+       if (ret < 0)
+               return ret;
+
+       idr_set_cursor(&pid_ns->idr, pid_ns->idr.idr_next + 1);
+       return 0;
 }
 
 extern int pid_max;


> 
> Thanks,
> Gargi
> >
> > Oleg.
> >