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. I was going to do some quick measurements of this, but the code oopses on power4 (http://userweb.kernel.org/~akpm/s5000402.jpg) 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? 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. 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"? - 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
