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    SubjectRe: [PATCH 1/4] mm: Trial do_wp_page() simplification
    On Thu, Sep 17, 2020 at 07:09:00PM -0300, Jason Gunthorpe wrote:
    > On Thu, Sep 17, 2020 at 05:40:59PM -0400, Peter Xu wrote:
    > > On Thu, Sep 17, 2020 at 01:35:56PM -0700, Linus Torvalds wrote:
    > > > For that to happen, we'd need to have the vma flag so that we wouldn't
    > > > have any worry about non-pinners, but as you suggested, I think even
    > > > just a mm-wide counter - or flag - to deal with the fast-bup case is
    > > > likely perfectly sufficient.
    > >
    > > Would mm_struct.pinned_vm suffice?
    >
    > I think that could be a good long term goal
    >
    > IIRC last time we dug into the locked_vm vs pinned_vm mess it didn't
    > get fixed. There is a mix of both kinds, as you saw, and some
    > resistance I don't clearly remember to changing it.
    >
    > My advice for this -rc fix is to go with a single bit in the mm_struct
    > set on any call to pin_user_pages*
    >
    > Then only users using pin_user_pages and forking are the only ones who
    > would ever do extra COW on fork. I think that is OK for -rc, this
    > workload should be rare due to the various historical issues. Anyhow,
    > a slow down regression is better than a it is broken regression.
    >
    > This can be improved into a counter later. Due to the pinned_vm
    > accounting all call sites should have the mm_struct at unpin, but I
    > have a feeling it will take a alot of driver patches to sort it all
    > out.

    Agreed. The HFI1 driver for example increments/decrements pinned_vm on it's
    own. I've kind of always felt dirty for that...

    I think long term it would be better to move this accounting to
    pin_user_pages() but Jason is correct that I think that is going to be too
    complex for an rc.

    Could we move pinned_vm out of the drivers/rdma subsystem?

    Ira

    >
    > Jason
    >

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