Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Jerome Glisse]

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 ?

Cheers,
Jérôme