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    SubjectRe: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()
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    On 07/01/2018 11:34 PM, Leon Romanovsky wrote:
    > On Sun, Jul 01, 2018 at 11:10:04PM -0700, John Hubbard wrote:
    >> On 07/01/2018 10:52 PM, Leon Romanovsky wrote:
    >>> On Thu, Jun 28, 2018 at 11:17:43AM +0200, Jan Kara wrote:
    >>>> On Wed 27-06-18 19:42:01, John Hubbard wrote:
    >>>>> On 06/27/2018 10:02 AM, Jan Kara wrote:
    >>>>>> On Wed 27-06-18 08:57:18, Jason Gunthorpe wrote:
    >>>>>>> On Wed, Jun 27, 2018 at 02:42:55PM +0200, Jan Kara wrote:
    >>>>>>>> On Wed 27-06-18 13:59:27, Michal Hocko wrote:
    >>>>>>>>> On Wed 27-06-18 13:53:49, Jan Kara wrote:
    >>>>>>>>>> On Wed 27-06-18 13:32:21, Michal Hocko wrote:
    >>>>>>>>> [...]
    >>>>> One question though: I'm still vague on the best actions to take in the
    >>>>> following functions:
    >>>>>
    >>>>> page_mkclean_one
    >>>>> try_to_unmap_one
    >>>>>
    >>>>> At the moment, they are both just doing an evil little early-out:
    >>>>>
    >>>>> if (PageDmaPinned(page))
    >>>>> return false;
    >>>>>
    >>>>> ...but we talked about maybe waiting for the condition to clear, instead?
    >>>>> Thoughts?
    >>>>
    >>>> What needs to happen in page_mkclean() depends on the caller. Most of the
    >>>> callers really need to be sure the page is write-protected once
    >>>> page_mkclean() returns. Those are:
    >>>>
    >>>> pagecache_isize_extended()
    >>>> fb_deferred_io_work()
    >>>> clear_page_dirty_for_io() if called for data-integrity writeback - which
    >>>> is currently known only in its caller (e.g. write_cache_pages()) where
    >>>> it can be determined as wbc->sync_mode == WB_SYNC_ALL. Getting this
    >>>> information into page_mkclean() will require some plumbing and
    >>>> clear_page_dirty_for_io() has some 50 callers but it's doable.
    >>>>
    >>>> clear_page_dirty_for_io() for cleaning writeback (wbc->sync_mode !=
    >>>> WB_SYNC_ALL) can just skip pinned pages and we probably need to do that as
    >>>> otherwise memory cleaning would get stuck on pinned pages until RDMA
    >>>> drivers release its pins.
    >>>
    >>> Sorry for naive question, but won't it create too much dirty pages
    >>> so writeback will be called "non-stop" to rebalance watermarks without
    >>> ability to progress?
    >>>
    >>
    >> That is an interesting point.
    >>
    >> Holding off page writeback of this region does seem like it could cause
    >> problems under memory pressure. Maybe adjusting the watermarks so that we
    >> tell the writeback system, "all is well, just ignore this region until
    >> we're done with it" might help? Any ideas here are welcome...
    >
    > AFAIR, it is per-zone, so the solution to count dirty-but-untouchable
    > number of pages to take them into account for accounting can work, but
    > it seems like an overkill. Can we create special ZONE for such gup
    > pages, or this is impossible too?
    >

    Let's see what Michal and others prefer. The zone idea intrigues me.

    thanks,
    --
    John Hubbard
    NVIDIA

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