On Thu, Nov 20, 2008 at 08:27:19AM -0700, Matthew Wilcox wrote: > I'm not quite sure why you need kmap_coherent(). If a page is mapped into > userspace, you can find what address it's mapped to from > page->mapping->i_mmap and page->index. OTOH, that's potentially Even if we know the userspace address of a page we do not necessarily have a usable mapping for kernel purposes. The userspace mapping might be r/o when we need r/w or it might be in another process. kmap_coherent takes the job of creating a r/w mapping on a suitable kernel virtual address that will avoid any aliases. > page->mapping->i_mmap and page->index. OTOH, that's potentially > expensive since you need to grab the spinlock, and unless you have all > user addresses coherent with each other (like parisc does), you need to > figure out which process to be coherent with. Having all userspace addresses of a page across all processes coherent with each other is the only practicable solution in Linux; at least I don't think how otherwise and within the currently kernel framework a platform could sanely handle userspace-userspace aliases. So we're talking about extending this to cover userspace-kernelspace aliases. The original reason for the introduction of kmap_coherent was avoiding a cache alias in when a multi-threaded process forks. The issue has been debated on lkml in 2006 as part of my submission of a patchset under the subject of "Fix COW D-cache aliasing on fork". The description is somewhat lengthy so I omit it here. One of the ugly parts of kmap_coherent() is that it cannot be used safely if the page has been marked as dirty by flush_dcache_page(); the callers know about this and deal with it. > I know James Bottomley did an experiment (and did an OLS presentation > ...) on unmapping the entire page cache and greatly expanding the kmap > area to do just this kind of thing. I think he even got a speedup. The speedup is no surprise. Ralf -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
