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@xxxxxxxxxx>.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> 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@xxxxxxxxxx>
> 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
