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    SubjectRe: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
    On Mon, Dec 17, 2018 at 03:55:01PM -0500, Jerome Glisse wrote:
    > On Mon, Dec 17, 2018 at 11:59:22AM -0800, Matthew Wilcox wrote:
    > > On Mon, Dec 17, 2018 at 02:54:08PM -0500, Jerome Glisse wrote:
    > > > On Mon, Dec 17, 2018 at 11:51:51AM -0800, Matthew Wilcox wrote:
    > > > > On Mon, Dec 17, 2018 at 02:48:00PM -0500, Jerome Glisse wrote:
    > > > > > On Mon, Dec 17, 2018 at 10:34:43AM -0800, Matthew Wilcox wrote:
    > > > > > > No. The solution John, Dan & I have been looking at is to take the
    > > > > > > dirty page off the LRU while it is pinned by GUP. It will never be
    > > > > > > found for writeback.
    > > > > >
    > > > > > With the solution you are proposing we loose GUP fast and we have to
    > > > > > allocate a structure for each page that is under GUP, and the LRU
    > > > > > changes too. Moreover by not writing back there is a greater chance
    > > > > > of data loss.
    > > > >
    > > > > Why can't you store the hmm_data in a side data structure? Why does it
    > > > > have to be in struct page?
    > > >
    > > > hmm_data is not even the issue here, we can have a pincount without
    > > > moving things around. So i do not see the need to complexify any of
    > > > the existing code to add new structure and consume more memory for
    > > > no good reasons. I do not see any benefit in that.
    > >
    > > You said "we have to allocate a structure for each page that is under
    > > GUP". The only reason to do that is if we want to keep hmm_data in
    > > struct page. If we ditch hmm_data, there's no need to allocate a
    > > structure, and we don't lose GUP fast either.
    >
    > And i have propose a way that do not need to ditch hmm_data nor
    > needs to remove page from the lru. What is it you do not like
    > with that ?

    I don't like bounce buffering. I don't like "end of writeback doesn't
    mark page as clean". I don't like pages being on the LRU that aren't
    actually removable. I don't like writing pages back which we know we're
    going to have to write back again.

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