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Message-Id: <20230614133337.f5eb6ef530d7d6cb1edbcf0d@linux-foundation.org>
Date: Wed, 14 Jun 2023 13:33:37 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/5] mmzone: Introduce folio_migratetype()
On Wed, 14 Jun 2023 21:17:34 +0100 Matthew Wilcox <willy@...radead.org> wrote:
> On Wed, Jun 14, 2023 at 01:13:05PM -0700, Andrew Morton wrote:
> > On Tue, 13 Jun 2023 19:13:09 -0700 "Vishal Moola (Oracle)" <vishal.moola@...il.com> wrote:
> >
> > > Introduce folio_migratetype() as a folio equivalent for
> > > get_pageblock_migratetype(). This function intends to return the
> > > migratetype the folio is located in, hence the name choice.
> > >
> > > ...
> > >
> > > --- a/include/linux/mmzone.h
> > > +++ b/include/linux/mmzone.h
> > > @@ -105,6 +105,9 @@ extern int page_group_by_mobility_disabled;
> > > #define get_pageblock_migratetype(page) \
> > > get_pfnblock_flags_mask(page, page_to_pfn(page), MIGRATETYPE_MASK)
> > >
> > > +#define folio_migratetype(folio) \
> > > + get_pfnblock_flags_mask(&folio->page, folio_pfn(folio), \
> > > + MIGRATETYPE_MASK)
> >
> > Theoretically this is risky because it evaluates its argument more than
> > once. Although folio_migratetype(folio++) seems an unlikely thing to do.
>
> folio++ is always an unsafe thing to do. folios are not consecutive
> in memory (unless we know they're order-0).
OK, folio_migratetype(expensive_function_which_returns_a_folio()) or
folio_migratetype(function_with_side_effects_which_returns_a_folio()).
There are many failure modes.
> > An inlined C function is always preferable. My quick attempt at that
> > reveals that the header files are All Messed Up As Usual.
>
> The page-equivalent of this also evaluates its arguments more than once,
> so it doesn't see too risky for now?
We got lucky. It's just bad practice.
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