Re: [PATCH v5 00/25] fs/dax: Fix ZONE_DEVICE page reference counts

[Dan Williams <dan.j.williams@intel.com>]

Andrew Morton wrote:
> On Tue,  7 Jan 2025 14:42:16 +1100 Alistair Popple <apopple@nvidia.com> wrote:
> 
> > Device and FS DAX pages have always maintained their own page
> > reference counts without following the normal rules for page reference
> > counting. In particular pages are considered free when the refcount
> > hits one rather than zero and refcounts are not added when mapping the
> > page.
> > 
> > Tracking this requires special PTE bits (PTE_DEVMAP) and a secondary
> > mechanism for allowing GUP to hold references on the page (see
> > get_dev_pagemap). However there doesn't seem to be any reason why FS
> > DAX pages need their own reference counting scheme.
> > 
> > By treating the refcounts on these pages the same way as normal pages
> > we can remove a lot of special checks. In particular pXd_trans_huge()
> > becomes the same as pXd_leaf(), although I haven't made that change
> > here. It also frees up a valuable SW define PTE bit on architectures
> > that have devmap PTE bits defined.
> > 
> > It also almost certainly allows further clean-up of the devmap managed
> > functions, but I have left that as a future improvment. It also
> > enables support for compound ZONE_DEVICE pages which is one of my
> > primary motivators for doing this work.
> > 
> 
> https://lkml.kernel.org/r/wysuus23bqmjtwkfu3zutqtmkse3ki3erf45x32yezlrl24qto@xlqt7qducyld
> made me expect merge/build/runtime issues, however this series merges
> and builds OK on mm-unstable.  Did something change?  What's the story
> here?
> 
> Oh well, it built so I'll ship it!

So my plan is to review this latest set on top of -next as is and then
rebase (or ask Alistair to rebase) on a mainline tag so I can identify
the merge conflicts with -mm and communicate those to Linus.

I will double check that you have pulled these back out of mm-unstable
before doing that to avoid a double-commit conflicts in -next, but for
now exposure in mm-unstable is good to flush out issues.