Messages in this thread | | | Date | Sun, 6 Apr 2003 00:06:21 +0200 | From | Andrea Arcangeli <> | Subject | Re: objrmap and vmtruncate |
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On Sat, Apr 05, 2003 at 01:24:06PM -0800, Andrew Morton wrote: > Andrea Arcangeli <andrea@suse.de> wrote: > > > > On Sat, Apr 05, 2003 at 04:06:14AM -0800, Andrew Morton wrote: > > > Andrew Morton <akpm@digeo.com> wrote: > > > > > > > > Nobody has written an "exploit" for this yet, but it's there. > > > > > > Here we go. The test app is called `rmap-test'. It is in ext3 CVS. See > > > > > > http://www.zip.com.au/~akpm/linux/ext3/ > > > > > > It sets up N MAP_SHARED VMA's and N tasks touching them in various access > > > patterns. > > > > I'm not questioning during paging rmap is more efficient than objrmap, > > but your argument about rmap having lower complexity of objrmap and that > > rmap is needed is wrong. The fact is that with your 100 mappings per > > each of the 100 tasks case, both algorithms works in O(N) where N is > > the number of the pagetables mapping the page. > > Nope. To unmap a page, full rmap has to scan 100 pte_chain slots, which is 3 > cachelines worth. objrmap has to scan 10,000 vma's, 9,900 of which do not map > that page at all.
I see what you mean, you're right. That's because all the 10,000 vma belongs to the same inode.
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