On Sun, Feb 04, 2007 at 01:44:45AM -0800, Andrew Morton wrote: > On Sun, 4 Feb 2007 09:51:07 +0100 (CET) Nick Piggin <npiggin@xxxxxxx> wrote: > > > 2. If we find the destination page is non uptodate, unlock it (this could be > > made slightly more optimal), then find and pin the source page with > > get_user_pages. Relock the destination page and continue with the copy. > > However, instead of a usercopy (which might take a fault), copy the data > > via the kernel address space. > > argh. We just can't go adding all this gunk into the write() path. > > mmap_sem, a full pte-walk, taking of pte-page locks, etc. For every page. > Even single-process write() will suffer, let along multithreaded stuff, > where mmap_sem contention may be the bigger problem. The write path is broken. I prefer my kernels slow, than buggy. As I said, I'm working on a replacement API so that the filesystems that care, can be correct *and* fast. > I was going to do some quick measurements of this, but the code oopses > on power4 (http://userweb.kernel.org/~akpm/s5000402.jpg) Cool, a kernel thread is calling sys_write. Fun. I guess I should be able to reproduce this if using initramfs. Thanks. > There's a build error in filemap_xip.c btw. > > > > We need to think different. > > What happened to the idea of doing an atomic copy into the non-uptodate > page and handling it somehow? That was my second idea. I didn't get any feedback on that patchset except to try this method, so I assume everyone hated it. I actually liked it, because it didn't have to do the writev segment-at-a-time for !uptodate pages like this one does. Considering this code gets called from mm-less contexts, maybe I'll have to go back to this approach. > Another option might be to effectively pin the whole mm during the copy: > > down_read(¤t->mm->unpaging_lock); > get_user(addr); /* Fault the page in */ > ... > copy_from_user() > up_read(¤t->mm->unpaging_lock); > > then, anyone who wants to unmap pages from this mm requires > write_lock(unpaging_lock). So we know the results of that get_user() > cannot be undone. Fugly. Don't know whether there are any lock order problems making it hard to implement, but you introduce the theoretical memory deadlock where a task cannot reclaim its own memory. > Or perhaps something like this can be done on a per-vma basis. Just > something to tell the VM "hey, you're not allowed to unmap this page right > now"? Same memory deadlock problem. - 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
