Re: get_user_pages() and shared memory question

[Roland Dreier <roland@topspin.com>]

    Timur> Hi, Is it possible for a page of memory that's been
    Timur> "grabbed" with get_user_pages() to ever be allocated to
    Timur> another process?  I'm assuming the answer is no, but I have
    Timur> a specific case I want to ask about.

Not to the best of my knowledge, although you should probably read the
code to convince yourself.

    Timur> Until 2.6.7, there was a bug in the VM where a page that
    Timur> was grabbed with get_user_pages() could be swapped out.
    Timur> Those of you familar with the OpenIB work know what I'm
    Timur> talking about.  Would that bug affect anything I'm talking
    Timur> about?

This isn't what the bug caused.  What could happen was that the
swapper could unmap a page from a process's virtual memory map before
it noticed that the page had an elevated reference count.  The page
wouldn't get swapped out, but when the process caused a page fault to
bring the virtual address back, it would get a different piece of
physical memory.

 - R.