linux-kernel - Re: [PATCH v11 2/3] ipc: Conserve sequence numbers in ipcmni

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Message-ID: <f09342e2-e040-5d8d-e1c5-5ff60d913505@redhat.com>
Date:   Sat, 10 Nov 2018 08:55:00 -0500
From:   Waiman Long <longman@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Takashi Iwai <tiwai@...e.de>, Davidlohr Bueso <dbueso@...e.de>,
        Manfred Spraul <manfred@...orfullife.com>
Subject: Re: [PATCH v11 2/3] ipc: Conserve sequence numbers in ipcmni_extend
 mode

On 11/10/2018 02:41 AM, Matthew Wilcox wrote:
> On Fri, Nov 09, 2018 at 03:11:31PM -0500, Waiman Long wrote:
>> The mixing in of a sequence number into the IPC IDs is probably to
>> avoid ID reuse in userspace as much as possible. With ipcmni_extend
>> mode, the number of usable sequence numbers is greatly reduced leading
>> to higher chance of ID reuse.
>>
>> To address this issue, we need to conserve the sequence number space
>> as much as possible. Right now, the sequence number is incremented
>> for every new ID created. In reality, we only need to increment the
>> sequence number when one or more IDs have been removed previously to
>> make sure that those IDs will not be reused when a new one is built.
>> This is being done irrespective of the ipcmni mode.
> That's not what I said.  Increment the sequence ID when the cursor wraps,
> not when there's been a deletion.

With non-cyclic idr allocation, the cursor will never wraps back to 0.
It is to the lowest available integer. I can do that with cyclic idr
allocation.

Cheers,
Longman