On Fri, 27 Jan 2012, Roland Dreier wrote: > > 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 My first impression is that that's not what you want at all: that will not do a preliminary COW of anonymous page to be written into by the driver when the user only wants VM_READ access. But perhaps I'm worrying about the second order while you're sticking to first order. Or perhaps I'm misunderstanding the context in which you want to do this: are you now accepting to do a different get_user_pages in the anonymous and driver-memory cases, and this suggestion was for the driver-memory case only? > (or is it VMA_MAYWRITE / force==1? > I don't quite understand the distinction between WRITE and MAYWRITE). I may have told you more than you wanted to know in the other mail. > > 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? I cannot go further, without explanation for why you need get_user_pages in the driver-memory case at all. > > Am I all wet, or am I becoming an MM hacker? Certainly more than I'll ever be an RDMA hacker, Hugh