On Thu, Nov 20, 2008 at 05:17:14PM +0000, Ralf Baechle wrote: > 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. Ah, I didn't do a good enough job of explaining. My question was why you needed the 'address' argument to kmap_coherent (and thus had a different prototype from kmap) instead of just implementing kmap() on mips. > 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. If you fix kmap() to only give you addresses that are coherent with userspace, you no longer need flush_dcache_page(). -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." -- 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
