[PATCH 7/8] mm: remove the extra ZONE_DEVICE struct page refcount

[Jason Gunthorpe]

On Mon, Feb 07, 2022 at 07:32:48AM +0100, Christoph Hellwig wrote:
> 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 pages.
> 
> Note that this excludes the special idle page wakeup for fsdax pages,
> which still happens at refcount 1.  This is a separate issue and will
> be sorted out later.  Given that only fsdax pages require the
> notifiacation when the refcount hits 1 now, the PAGEMAP_OPS Kconfig
> symbol can go away and be replaced with a FS_DAX check for this hook
> in the put_page fastpath.
> 
> Based on an earlier patch from Ralph Campbell <rcampbell at nvidia.com>.
> 
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> ---
>  arch/powerpc/kvm/book3s_hv_uvmem.c       |  1 -
>  drivers/gpu/drm/amd/amdkfd/kfd_migrate.c |  1 -
>  drivers/gpu/drm/nouveau/nouveau_dmem.c   |  1 -
>  fs/Kconfig                               |  1 -
>  include/linux/memremap.h                 | 12 +++--
>  include/linux/mm.h                       |  6 +--
>  lib/test_hmm.c                           |  1 -
>  mm/Kconfig                               |  4 --
>  mm/internal.h                            |  2 +
>  mm/memcontrol.c                          | 11 ++---
>  mm/memremap.c                            | 57 ++++++++----------------
>  mm/migrate.c                             |  6 ---
>  mm/swap.c                                | 16 ++-----
>  13 files changed, 36 insertions(+), 83 deletions(-)

It looks like a good next step to me

Reviewed-by: Jason Gunthorpe <jgg at nvidia.com>

>  struct dev_pagemap_ops {
>  	/*
> -	 * Called once the page refcount reaches 1.  (ZONE_DEVICE pages never
> -	 * reach 0 refcount unless there is a refcount bug. This allows the
> -	 * device driver to implement its own memory management.)
> +	 * Called once the page refcount reaches 0.  The reference count will be
> +	 * reset to one by the core code after the method is called to prepare
> +	 * for handing out the page again.

I did prefer Ralph's version of this that kept the refcount at 0 while
the page was on the free-list. I hope we can get there again after
later series :)

Jason