On Wed, May 30, 2007 at 02:27:52PM -0700, Linus Torvalds wrote: > Well, don't think of it as a special case at all: think of bit 30 as a > "the user asked for a non-linear fd". > In fact, to make it effective, I'd suggest literally scrambling the low > bits (using, for example, some silly per-boot xor value to to actually > generate the "true" index - the equivalent of a really stupid randomizer). > That way you'd have the legacy "linear" space, and a separate "non-linear > space" where people simply *cannot* make assumptions about contiguous fd > allocations. There's no special case there - it's just an extension which > explicitly allows us to say "if you do that, your fd's won't be allocated > the traditional way any more, but you *can* mix the traditional and the > non-linear allocation".
One could always stuff a seed or per-cpu seeds in the files_struct and use a PRNG. The only trick would be cacheline bounces and/or space consumption of seeds. Another possibility would be bitreversed contiguity or otherwise a bit permutation of some contiguous range, modulo (of course) the high bit used to tag the randomized range. With "truly" random/sparse fd numbers it may be meaningful to use a different data structure from a bitmap to track them in-kernel, though xor and other easily-computed mappings to/from contiguous ranges won't need such in earnest. -- wli - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
