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    SubjectRe: [PATCH RFC] vm_unmap_aliases: allow callers to inhibit TLB flush
    Nick Piggin wrote:
    > On Friday 20 February 2009 06:11:32 Jeremy Fitzhardinge wrote:
    >> Nick Piggin wrote:
    >>> Then what is the point of the vm_unmap_aliases? If you are doing it
    >>> for security it won't work because other CPUs might still be able
    >>> to write through dangling TLBs. If you are not doing it for
    >>> security then it does not need to be done at all.
    >> Xen will make sure any danging tlb entries are flushed before handing
    >> the page out to anyone else.
    >>> Unless it is something strange that Xen does with the page table
    >>> structure and you just need to get rid of those?
    >> Yeah. A pte pointing at a page holds a reference on it, saying that it
    >> belongs to the domain. You can't return it to Xen until the refcount is 0.
    > OK. Then I will remember to find some time to get the interrupt
    > safe patches working. I wonder why you can't just return it to
    > Xen when (or have Xen hold it somewhere until) the refcount
    > reaches 0?

    It would still need to allocate a page in the meantime, which could fail
    because the domain has hit its hard memory limit (which will be the
    common case, because a domain generally starts with its full compliment
    of memory). The nice thing about the exchange is that there's no
    accounting to take into account.

    >>> Or... what if we just allow a compile and/or boot time flag to direct
    >>> that it does not want lazy vmap unmapping and it will just revert to
    >>> synchronous unmapping? If Xen needs lots of flushing anyway it might
    >>> not be a win anyway.
    >> That may be worth considering.
    > ... in the meantime, shall we just do this for Xen? It is probably
    > safer and may end up with no worse performance on Xen anyway. If
    > we get more vmap users and it becomes important, you could look at
    > more sophisticated ways of doing this. Eg. a page could be flagged
    > if it potentially has lazy vmaps.

    OK. Do you want to do the patch, or shall I?


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