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

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Message-ID: <20190912211114.GA146974@google.com>
Date:   Thu, 12 Sep 2019 15:11:14 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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 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.

> Yes, we have one spare bit between order and number of object that is not
> used and always zero. So what?
> 
> I can imagine for some microarchitecures accessing higher 16 bits of int
> is cheaper than shifting by 15.

Well, I highly doubt the inconsistency is intended for such
optimization, even it existed.