Re: [PATCH v1 02/14] mm: remove extra ZONE_DEVICE struct page refcount

[Ralph Campbell <rcampbell@nvidia.com>]

On 8/25/21 4:15 AM, Vlastimil Babka wrote:
> On 8/25/21 05:48, 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.
> That's certainly welcome. I just wonder what was the reason to use 1 in the
> first place and why it's no longer necessary?

I'm sure this is a long story that I don't know most of the history.
I'm guessing that ZONE_DEVICE started out with a reference count of
one since that is what most "normal" struct page pages start with.
Then put_page() is used to free newly initialized struct pages to the
slab/slob/slub page allocator.
This made it easy to call get_page()/put_page() on ZONE_DEVICE pages
since get_page() asserts that the caller has a reference.
However, most drivers that create ZONE_DEVICE struct pages just insert
a PTE into the user page tables and don't increment/decrement the
reference count. MEMORY_DEVICE_FS_DAX used the >1 to 1 reference count
transition to signal that a page was idle so that made put_page() a
bit more complex. Then MEMORY_DEVICE_PRIVATE pages were added and this
special casing of what "idle" meant got more complicated and more parts
of mm had to check for is_device_private_page().
My goal was to make ZONE_DEVICE struct pages reference counts be zero
based and allocated/freed similar to the page allocator so that more
of the mm code could be used, like THP ZONE_DEVICE pages, without special
casing ZONE_DEVICE.