linux-kernel - Re: [patch 02/20] posix-timers: Ensure timer ID search-loop limit is valid

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Message-ID: <87v8h126p2.ffs@tglx>
Date:   Tue, 09 May 2023 11:30:01 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        syzbot+5c54bd3eb218bb595aa9@...kaller.appspotmail.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Andrei Vagin <avagin@...nvz.org>
Subject: Re: [patch 02/20] posix-timers: Ensure timer ID search-loop limit
 is valid

On Mon, May 08 2023 at 23:57, Thomas Gleixner wrote:
> On Sat, May 06 2023 at 01:36, Thomas Gleixner wrote:
>> The only alternative solution I could come up with is a paritioned
>> xarray where the index space would be segmented for each TGID, i.e.
>>
>>        segment.start = TGID * MAX_TIMERS_PER_PROCESS
>>        segment.end    = segment.start + MAX_TIMERS_PER_PROCESS - 1
>>
>> where MAX_TIMERS_PER_PROCESS could be a copius 2^16 which would work for
>> both 32bit and 64bit TID limits.
>>
>> That would avoid the hash table lookups and the related issues, but OTH
>> it would require to allocate one extra page per TGID if the application
>> uses a single posix timer.
>>
>> Not sure whether that's worth it though.
>
> More thoughts on this. If we go there and accept the extra page of
> memory then we can just go all the way and make the xarray per process,
> actually per signal.

Thinking more about it. The current scheme how timer ID allocation works
is really interesting vs. CRIU.

Assume a process creates/deletes timers frequently. It's not hard to
move the next ID close to INT_MAX, i.e. 2G

Now checkpoint that thing and restore it which means to do the
create/delete dance to move next ID up to the last one-1. Will only take
a couple of hours....

Thanks,

        tglx