> Sigh, what a mess ... it seems what we really want to do is know > if userspace might trigger a COW because or not, and only do a > preemptive COW in that case. (We're not really concerned with > userspace fork()ing and setting up a COW in the future, since that's > what we have MADV_DONTFORK for) > > The status quo works for userspace anonymous mappings but > it doesn't work for my case of mapping a kernel buffer read-only > into userspace. And fixing my case breaks the anonymous case. > Do you see a way out of this dilemma? Do we need to add yet > another flag to get_user_pages()? So thinking about this a bit more... it seems what we want is at least to first order that we do the equivalent of write==1 exactly when the vma for a mapping has VM_WRITE set (or is it VMA_MAYWRITE / force==1? I don't quite understand the distinction between WRITE and MAYWRITE). Right now, one call to get_user_pages() might involve more than one vma, but we could simulate the above by doing find_vma() and making sure our call to get_user_pages() goes one vma at a time. Of course that would be inefficient since get_user_pages() will redo the find_vma() internally, so it would I guess make sense to add another FOLL_ flag to tell get_user_pages() to do this? Am I all wet, or am I becoming an MM hacker? Thanks, Roland -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href