On Wed, Oct 20, 2021 at 12:44 PM Catalin Marinas
<catalin.marinas@xxxxxxx> wrote:
>
> However, with MTE doing both get_user() every 16 bytes and
> gup can get pretty expensive.
So I really think that anything that is performance-critical had
better only do the "fault_in_write()" code path in the cold error path
where you took a page fault.
IOW, the loop structure should be
repeat:
take_locks();
pagefault_disable();
ret = do_things_with_locks();
pagefault_enable();
release_locks();
// Failure path?
if (unlikely(ret == -EFAULT)) {
if (fault_in_writable(..))
goto repeat;
return -EFAULT;
}
rather than have it be some kind of "first do fault_in_writable(),
then do the work" model.
So I wouldn't worry too much about the performance concerns. It simply
shouldn't be a common or hot path.
And yes, I've seen code that does that "fault_in_xyz()" before the
critical operation that cannot take page faults, and does it
unconditionally.
But then it isn't the "fault_in_xyz()" that should be blamed if it is
slow, but the caller that does things the wrong way around.
Linus
