[PATCH v3 2/8] mm: remove extra ZONE_DEVICE struct page refcount

[Ralph Campbell]

On 6/17/21 8:16 AM, Alex Sierra wrote:
> From: Ralph Campbell <rcampbell at nvidia.com>
>
> ZONE_DEVICE struct pages have an extra reference count that complicates the
> code for put_page() and several places in the kernel that need to check the
> reference count to see that a page is not being used (gup, compaction,
> migration, etc.). Clean up the code so the reference count doesn't need to
> be treated specially for ZONE_DEVICE.
>
> v2:
> AS: merged this patch in linux 5.11 version
>
> Signed-off-by: Ralph Campbell <rcampbell at nvidia.com>
> Signed-off-by: Alex Sierra <alex.sierra at amd.com>
> ---
>   arch/powerpc/kvm/book3s_hv_uvmem.c     |  2 +-
>   drivers/gpu/drm/nouveau/nouveau_dmem.c |  2 +-
>   fs/dax.c                               |  4 +-
>   include/linux/dax.h                    |  2 +-
>   include/linux/memremap.h               |  7 +--
>   include/linux/mm.h                     | 44 -----------------
>   lib/test_hmm.c                         |  2 +-
>   mm/internal.h                          |  8 +++
>   mm/memremap.c                          | 68 +++++++-------------------
>   mm/migrate.c                           |  5 --
>   mm/page_alloc.c                        |  3 ++
>   mm/swap.c                              | 45 ++---------------
>   12 files changed, 45 insertions(+), 147 deletions(-)
>
I think it is great that you are picking this up and trying to revive it.

However, I have a number of concerns about how it affects existing ZONE_DEVICE
MEMORY_DEVICE_GENERIC and MEMORY_DEVICE_FS_DAX users and I don't see this
addressing them. For example, dev_dax_probe() allocates MEMORY_DEVICE_GENERIC
struct pages and then:
   dev_dax_fault()
     dev_dax_huge_fault()
       __dev_dax_pte_fault()
         vmf_insert_mixed()
which just inserts the PFN into the CPU page tables without increasing the page
refcount so it is zero (whereas it was one before). But using get_page() will
trigger VM_BUG_ON_PAGE() if it is enabled. There isn't any current notion of
free verses allocated for these struct pages. I suppose init_page_count()
could be called on all the struct pages in dev_dax_probe() to fix that though.

I'm even less clear about how to fix MEMORY_DEVICE_FS_DAX. File systems have clear
allocate and free states for backing storage but there are the complications with
the page cache references, etc. to consider. The >1 to 1 reference count seems to
be used to tell when a page is idle (no I/O, reclaim scanners) rather than free
(not allocated to any file) but I'm not 100% sure about that since I don't really
understand all the issues around why a file system needs to have a DAX mount option
besides knowing that the storage block size has to be a multiple of the page size.