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    SubjectRe: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address
    On Fri, Mar 08, 2019 at 05:13:26PM +0800, Jason Wang wrote:
    > Actually not wrapping around,  the pages for used ring was marked as
    > dirty after a round of virtqueue processing when we're sure vhost wrote
    > something there.

    Thanks for the clarification. So we need to convert it to
    set_page_dirty and move it to the mmu notifier invalidate but in those
    cases where gup_fast was called with write=1 (1 out of 3).

    If using ->invalidate_range the page pin also must be removed
    immediately after get_user_pages returns (not ok to hold the pin in
    vmap until ->invalidate_range is called) to avoid false positive gup
    pin checks in things like KSM, or the pin must be released in
    invalidate_range_start (which is called before the pin checks).

    Here's why:

    /*
    * Check that no O_DIRECT or similar I/O is in progress on the
    * page
    */
    if (page_mapcount(page) + 1 + swapped != page_count(page)) {
    set_pte_at(mm, pvmw.address, pvmw.pte, entry);
    goto out_unlock;
    }
    [..]
    set_pte_at_notify(mm, pvmw.address, pvmw.pte, entry);
    ^^^^^^^ too late release the pin here, the
    above already failed

    ->invalidate_range cannot be used with mutex anyway so you need to go
    back with invalidate_range_start/end anyway, just the pin must be
    released in _start at the latest in such case.

    My prefer is generally to call gup_fast() followed by immediate
    put_page() because I always want to drop FOLL_GET from gup_fast as a
    whole to avoid 2 useless atomic ops per gup_fast.

    I'll write more about vmap in answer to the other email.

    Thanks,
    Andrea

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