linux-kernel - Re: [PATCH v2 1/4] mm: correct mask size for slub page->objects

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Message-ID: <20190912220303.ijdwnoxiwgv7mmv4@box>
Date:   Fri, 13 Sep 2019 01:03:03 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] mm: correct mask size for slub page->objects

On Thu, Sep 12, 2019 at 03:11:14PM -0600, Yu Zhao wrote:
> On Thu, Sep 12, 2019 at 12:40:35PM +0300, Kirill A. Shutemov wrote:
> > On Wed, Sep 11, 2019 at 08:31:08PM -0600, Yu Zhao wrote:
> > > Mask of slub objects per page shouldn't be larger than what
> > > page->objects can hold.
> > > 
> > > It requires more than 2^15 objects to hit the problem, and I don't
> > > think anybody would. It'd be nice to have the mask fixed, but not
> > > really worth cc'ing the stable.
> > > 
> > > Fixes: 50d5c41cd151 ("slub: Do not use frozen page flag but a bit in the page counters")
> > > Signed-off-by: Yu Zhao <yuzhao@...gle.com>
> > 
> > I don't think the patch fixes anything.
> 
> Technically it does. It makes no sense for a mask to have more bits
> than the variable that holds the masked value. I had to look up the
> commit history to find out why and go through the code to make sure
> it doesn't actually cause any problem.
> 
> My hope is that nobody else would have to go through the same trouble.

Just put some comments then.

-- 
 Kirill A. Shutemov