Re: [aarcange@redhat.com: [PATCH] fork vs gup(-fast) fix]

[Andrea Arcangeli <aarcange@redhat.com>]

On Fri, Mar 13, 2009 at 03:23:40AM +1100, Nick Piggin wrote:
> OK, this is as far as I got tonight.
> 
> This passes Andrea's dma_thread test case. I haven't started hugepages,
> and it isn't quite right to drop the mmap_sem and retake it for write
> in get_user_pages (firstly, caller might hold mmap_sem for write,
> secondly, it may not be able to tolerate mmap_sem being dropped).

What's the point? I mean this will simply work worse than my patch
because it'll have to don't-cow the whole range regardless if it's
pinned or not. Which will slowdown fork in the O_DIRECT case even
more, for no good reason. I thought the complaint here was only a
beauty issue of not wanting to add a function called fork_pre_cow or
your equivalent decow_one_pte in the fork path, not any practical
issue with my patch which already passed all sort of regression
testing and performance valuations. Plus you still have a per-page
bitflag, and I think you have implementation issues in the patch (the
parent pte can't be left writeable if you are in a don't-cow vma, or
the copy will not be atomic, and glibc will have no chance to fix its
bugs). You're not removing the fork_pre_cow logic from fork, so I can
only see it as a regression to make the logic less granular in the
vma.

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