linux-kernel - Re: [PATCH] zsmalloc: use U suffix for negative literals being shifted

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Message-ID: <20180110055338.h3cs5hw7mzsdtcad@eng-minchan1.roam.corp.google.com>
Date:   Wed, 10 Jan 2018 14:53:38 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Nick Desaulniers <nick.desaulniers@...il.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Nitin Gupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-mm <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] zsmalloc: use U suffix for negative literals being
 shifted

Hi Nick,

On Mon, Jan 08, 2018 at 08:35:19PM -0800, Nick Desaulniers wrote:
> On Sun, Jan 7, 2018 at 7:04 AM, Minchan Kim <minchan@...nel.org> wrote:
> > Sorry for the delay. I have missed this until now. ;-(
> 
> No worries, figured patches would need a post holiday bump for review.
> 
> >
> > On Sun, Dec 24, 2017 at 11:33 AM, Nick Desaulniers
> > <nick.desaulniers@...il.com> wrote:
> >> -                       link->next = -1 << OBJ_TAG_BITS;
> >> +                       link->next = -1U << OBJ_TAG_BITS;
> >
> > -1UL?
> 
> Oops, good catch.
> 
> > Please, resend it with including Andrew Morton
> > <akpm@...ux-foundation.org> who merges zsmalloc patch into his tree.
> 
> Will do.
> 
> On Sun, Jan 7, 2018 at 3:02 PM, Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
> > Oh, boy, shouldn't be rather GENMASK() / GENMASK_ULL() in a way how
> 
> Thanks for the suggestion. `GENMASK(BITS_PER_LONG - 1, OBJ_TAG_BITS);`
> is equivalent.  Whether that is more readable, I'll wait for Minchan
> to decide.  If that's preferred, I'll make sure to credit you with the
> Suggested-By tag in the commit message.

I don't see any benefit with GENMASK in our usecase.
If it's not a good justfication, I'd like to use just -1UL which
would be more readable without effort to understand new API.

Thanks.