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    SubjectRe: [PATCH v3 00/13] dax: fix dma vs truncate and remove 'page-less' support
    On Fri, Oct 20, 2017 at 09:47:50AM +0200, Christoph Hellwig wrote:
    > I'd like to brainstorm how we can do something better.
    >
    > How about:
    >
    > If we hit a page with an elevated refcount in truncate / hole puch
    > etc for a DAX file system we do not free the blocks in the file system,
    > but add it to the extent busy list. We mark the page as delayed
    > free (e.g. page flag?) so that when it finally hits refcount zero we
    > call back into the file system to remove it from the busy list.

    Brainstorming some more:

    Given that on a DAX file there shouldn't be any long-term page
    references after we unmap it from the page table and don't allow
    get_user_pages calls why not wait for the references for all
    DAX pages to go away first? E.g. if we find a DAX page in
    truncate_inode_pages_range that has an elevated refcount we set
    a new flag to prevent new references from showing up, and then
    simply wait for it to go away. Instead of a busy way we can
    do this through a few hashed waitqueued in dev_pagemap. And in
    fact put_zone_device_page already gets called when putting the
    last page so we can handle the wakeup from there.

    In fact if we can't find a page flag for the stop new callers
    things we could probably come up with a way to do that through
    dev_pagemap somehow, but I'm not sure how efficient that would
    be.

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