Re: [PATCH v6 02/13] mm: remove extra ZONE_DEVICE struct page refcount

[Ralph Campbell <rcampbell@nvidia.com>]

On 8/17/21 5:35 PM, Felix Kuehling wrote:
> Am 2021-08-17 um 8:01 p.m. schrieb Ralph Campbell:
>> On 8/12/21 11:31 PM, Alex Sierra wrote:
>>> From: Ralph Campbell <rcampbell@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
>>>
>>> v5:
>>> AS: add condition at try_grab_page to check for the zone device type,
>>> while
>>> page ref counter is checked less/equal to zero. In case of device
>>> zone, pages
>>> ref counter are initialized to zero.
>>>
>>> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
>>> Signed-off-by: Alex Sierra <alex.sierra@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                     | 13 +----
>>>    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, 46 insertions(+), 115 deletions(-)
>>>
>> I haven't seen a response to the issues I raised back at v3 of this
>> series.
>> https://lore.kernel.org/linux-mm/4f6dd918-d79b-1aa7-3a4c-caa67ddc29bc@nvidia.com/
>>
>>
>> Did I miss something?
> I think part of the response was that we did more testing. Alex added
> support for DEVICE_GENERIC pages to test_hmm and he ran DAX tests
> recommended by Theodore Tso. In that testing he ran into a WARN_ON_ONCE
> about a zero page refcount in try_get_page. The fix is in the latest
> version of patch 2. But it's already obsolete because John Hubbard is
> about to remove that function altogether.
>
> I think the issues you raised were more uncertainty than known bugs. It
> seems the fact that you can have DAX pages with 0 refcount is a feature
> more than a bug.
>
> Regards,
>    Felix

Did you test on a system without CONFIG_ARCH_HAS_PTE_SPECIAL defined?
In that case, mmap() of a DAX device will call insert_page() which calls
get_page() which would trigger VM_BUG_ON_PAGE().

I can believe it is OK for PTE_SPECIAL page table entries to have no
struct page or that MEMORY_DEVICE_GENERIC struct pages be mapped with
a zero reference count using insert_pfn().

I find it hard to believe that other MM developers don't see an issue
with a struct page with refcount == 0 and mapcount == 1.

I don't see where init_page_count() is being called for the
MEMORY_DEVICE_GENERIC or MEMORY_DEVICE_PRIVATE struct pages the AMD
driver allocates and passes to migrate_vma_setup().
Looks like svm_migrate_get_vram_page() needs to call init_page_count()
instead of get_page(). (I'm looking at branch origin/alexsierrag/device_generic
https://github.com/RadeonOpenCompute/ROCK-Kernel-Driver.git)

Also, what about the other places where is_device_private_page() is called?
Don't they need to be updated to call is_device_page() instead?
One of my goals for this patch was to remove special casing reference counts
for ZONE_DEVICE pages in rmap.c, etc.

I still think this patch needs an ACK from a FS/DAX maintainer.