Re: Corruption with O_DIRECT and unaligned user buffers

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Nov 19, 2008 at 03:25:59PM +1100, Nick Piggin wrote:
> The solution either involves synchronising forks and get_user_pages,
> or probably better, to do copy on fork rather than COW in the case
> that we detect a page is subject to get_user_pages. The trick is in
> the details :)

We already have a patch that works.

The only trouble here is get_user_pages_fast, it breaks the fix for
fork, the current ksm (that is safe against get_user_pages but can't
be safe against get_user_pages_fast) and even migrate.c
memory-corrupts against O_DIRECT after the introduction of
get_user_pages_fast.

So I recommend focusing on how to fix get_user_pages_fast for any of
the 3 broken pieces, then hopefully the same fix will work for the
other two.

fork is special in that it even breaks against get_user_pages but
again we've a fix for that. The only problem without a solution is how
to serialize against get_user_pages_fast. A brlock was my proposal,
not nice but still better than backing out get_user_pages_fast.