On Sun, 4 Feb 2007 09:51:07 +0100 (CET) Nick Piggin <[email protected]> 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"?
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