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    SubjectRe: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA
    On Wed, Feb 06, 2019 at 07:16:21PM +0000, Christopher Lameter wrote:
    > On Wed, 6 Feb 2019, Doug Ledford wrote:
    > > > Most of the cases we want revoke for are things like truncate().
    > > > Shouldn't happen with a sane system, but we're trying to avoid users
    > > > doing awful things like being able to DMA to pages that are now part of
    > > > a different file.
    > >
    > > Why is the solution revoke then? Is there something besides truncate
    > > that we have to worry about? I ask because EBUSY is not currently
    > > listed as a return value of truncate, so extending the API to include
    > > EBUSY to mean "this file has pinned pages that can not be freed" is not
    > > (or should not be) totally out of the question.
    > >
    > > Admittedly, I'm coming in late to this conversation, but did I miss the
    > > portion where that alternative was ruled out?
    >
    > Coming in late here too but isnt the only DAX case that we are concerned
    > about where there was an mmap with the O_DAX option to do direct write

    There is no O_DAX option. There's mount -o dax, but there's nothing that
    a program does to say "Use DAX".

    > though? If we only allow this use case then we may not have to worry about
    > long term GUP because DAX mapped files will stay in the physical location
    > regardless.

    ... except for truncate. And now that I think about it, there was a
    desire to support hot-unplug which also needed revoke.

    > Maybe we can solve the long term GUP problem through the requirement that
    > user space acquires some sort of means to pin the pages? In the DAX case
    > this is given by the filesystem and the hardware will basically take care
    > of writeback.

    It's not given by the filesystem.

    > In case of anonymous memory this can be guaranteed otherwise and is less
    > critical since these pages are not part of the pagecache and are not
    > subject to writeback.

    but are subject to being swapped out?

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