linux-kernel - Re: [PATCH 11/11] afs: Fix dirty-region encoding on ppc32 with 64K pages

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Message-ID: <20201028171159.GB20115@casper.infradead.org>
Date:   Wed, 28 Oct 2020 17:11:59 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-afs@...ts.infradead.org, kernel test robot <lkp@...el.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/11] afs: Fix dirty-region encoding on ppc32 with 64K
 pages

On Wed, Oct 28, 2020 at 05:05:08PM +0000, David Howells wrote:
> Matthew Wilcox <willy@...radead.org> wrote:
> 
> > > +{
> > > +	if (PAGE_SIZE - 1 <= __AFS_PAGE_PRIV_MASK)
> > > +		return 1;
> > > +	else
> > > +		return PAGE_SIZE / (__AFS_PAGE_PRIV_MASK + 1);
> > 
> > Could this be DIV_ROUND_UP(PAGE_SIZE, __AFS_PAGE_PRIV_MASK + 1); avoiding
> > a conditional?  I appreciate it's calculated at compile time today, but
> > it'll be dynamic with THP.
> 
> Hmmm - actually, I want a shift size, not a number of bytes as I divide by it
> twice in afs_page_dirty().

__AFS_PAGE_PRIV_MASK is a constant though.  If your compiler can't
optimise a divide-by-a-constant-power-of-two into a shift-by-a-constant (*),
it's time to get yourself a new compiler.

(*) assuming that's faster on the CPU it's targetting.