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From: Ira Weiny <ira.weiny@intel.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Alexander Duyck <alexander.duyck@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>, linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH] mm: Make PageType more efficient
Date: Wed, 11 Mar 2020 10:14:45 -0700	[thread overview]
Message-ID: <20200311171444.GA1446196@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <20200311131304.GD22433@bombadil.infradead.org>

On Wed, Mar 11, 2020 at 06:13:04AM -0700, Matthew Wilcox wrote:
> On Tue, Mar 10, 2020 at 02:50:50PM -0700, Alexander Duyck wrote:
> > On Tue, Mar 10, 2020 at 1:37 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > > > -#define PageType(page, flag)                                           \
> > > > > -       ((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
> > > > > -
> > 
> > >From what I can tell this is the only consumer of PAGE_TYPE_BASE.
> > Since it is removed you can probably remove that definition as well.
> 
> I _could_ ... I do want to indicate to people that they probably
> shouldn't use those bits in order to leave space for overflow and
> wraparound of _mapcount.
> 
> > > > > +#define PageType(page, flag)                                           \
> > > > > +       (page_has_type(page) && (~page->page_type & flag))
> > 
> > You can probably spare a cycle or two here by testing for
> > "!(page->page_type & flag)". That way you avoid the extra bit flipping
> > since the compiler can just handle the result of the AND op as it sees
> > fit.
> 
> GCC already knows to do that optimisation; mm/page_alloc.o is identical
> (same md5sum) when changing from (~page->page_type & flag) to
> !(page->page_type & flag).  So it's just a question of which one is

No dog in this fight...  But for simpletons like me...

!(page->page_type & flag)

... makes much more sense,
Ira

> easier for humans to read and reason about.  Do you have an opinion
> which one you'd like to see?
> 
> 


  reply	other threads:[~2020-03-11 17:14 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-10 18:56 [PATCH] mm: Make PageType more efficient Matthew Wilcox
2020-03-10 20:17 ` Alexander Duyck
2020-03-10 20:37   ` Matthew Wilcox
2020-03-10 21:50     ` Alexander Duyck
2020-03-11 13:13       ` Matthew Wilcox
2020-03-11 17:14         ` Ira Weiny [this message]
2020-03-11 17:22         ` Alexander Duyck
2020-03-11  1:10 ` Andrew Morton
2020-03-11 13:21   ` Matthew Wilcox

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