Messages in this thread | | | Date | Tue, 9 Aug 2011 08:03:52 -0700 (PDT) | From | Dan Magenheimer <> | Subject | RE: Subject: [PATCH V6 2/4] mm: frontswap: core code |
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> > +#ifndef CONFIG_FRONTSWAP > > +/* all inline routines become no-ops and all externs are ignored */ > > +#define frontswap_enabled (0) > > +#endif > > + > > +static inline int frontswap_test(struct swap_info_struct *sis, pgoff_t > > offset) > > +{ > > + int ret = 0; > > + > > + if (frontswap_enabled && sis->frontswap_map) > > + ret = test_bit(offset % BITS_PER_LONG, > > + &sis->frontswap_map[offset/BITS_PER_LONG]); > > if (sis->frontswap_map) > ret = test_bit(offset, sis->frontswap_map); > > (since sis->frontswap_map can't be non-NULL without > frontswap_enabled being true, and since test_bit() itself already > does what you open-coded here.
Hi Jan --
Thanks for the review!
> since test_bit() itself already does what you open-coded here
Good catch. Will change. Either is correct and I suspect the compiler may end up generating the same code, but your code is much more succinct.
> (since sis->frontswap_map can't be non-NULL without > frontswap_enabled being true
As noted in the comment immediately preceding, the frontswap_enabled check serves a second purpose: When CONFIG_FRONTSWAP is disabled, this entire inline function devolves to a compile-time constant (0), which avoids a handful of ifdef's in the core swap subsystem. (This approach was originally suggested for cleancache by Jeremy Fitzhardinge.)
Also, though this patch never unsets frontswap_enabled, it is a global and some future tmem backend might unset it, so it's probably best to leave the extra test anyway.
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