On Wed, Apr 04, 2007 at 01:11:11PM -0700, David Miller wrote: > From: Linus Torvalds <[EMAIL PROTECTED]> > Date: Wed, 4 Apr 2007 08:35:30 -0700 (PDT) > > > Anyway, I'm not against this, but I can see somebody actually *wanting* > > the ZERO page in some cases. I've used the fact for TLB testing, for > > example, by just doing a big malloc(), and knowing that the kernel will > > re-use the ZERO_PAGE so that I don't get any cache effects (well, at least > > not any *physical* cache effects. Virtually indexed cached will still show > > effects of it, of course, but I haven't cared). > > > > That's an example of an app that actually cares about the page allocation > > (or, in this case, the lack there-of). Not an important one, but maybe > > there are important ones that care? > > If we're going to consider this seriously, there is a case I know of. > Look at flush_dcache_page()'s test for ZERO_PAGE() on sparc64, there > is an instructive comment: > > /* Do not bother with the expensive D-cache flush if it > * is merely the zero page. The 'bigcore' testcase in GDB > * causes this case to run millions of times. > */ > if (page == ZERO_PAGE(0)) > return; > > basically what the GDB test case does it mmap() an enormous anonymous > area, not touch it, then dump core. > > As I understand the patch being considered to remove ZERO_PAGE(), this > kind of core dump will cause a lot of pages to be allocated, probably > eating up a lot of system time as well as memory.
Yeah. Well it is trivial to leave ZERO_PAGE in get_user_pages, however in the longer run it would be nice to get rid of ZERO_PAGE completely so we need an alternative. I've been working on a patch for core dumping that can detect unfaulted anonymous memory and skip it without doing the ZERO_PAGE comparision. - 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/
