From: Matthew Wilcox <willy@infradead.org>
To: Alexander Duyck <alexander.duyck@gmail.com>
Cc: 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 06:13:04 -0700 [thread overview]
Message-ID: <20200311131304.GD22433@bombadil.infradead.org> (raw)
In-Reply-To: <CAKgT0Uf+6AEpMCewGogowCf_24cimFq5ChmjneiA62kwFCS4Rg@mail.gmail.com>
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
easier for humans to read and reason about. Do you have an opinion
which one you'd like to see?
next prev parent reply other threads:[~2020-03-11 13:13 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 [this message]
2020-03-11 17:14 ` Ira Weiny
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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