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.