Page Migration

the old page is locked, and pte entries of the old page is replaced with migration entries for which the page fault with wait till the migration is finished

new page is locked and set not-uptodate before the migration so that processes will wait on the till the migration ends.

unmap old page with migration entries
replace the radix tree

if a page is locked then the processes trying to map that page in their pts will wait on the lock. also, the page won't be swapped out. 

there are two granularity of locking: 1. vma lock which is in virtual level lock, does not correspond to any physical pages. and 2. page lock which is the physical page lock.

page ref count means some process or kernel component is currently using or holding that physical page somehow. (page_count)

page map count means that page is mapped to some process page table (page_mapcount, +1)

map count <= ref count 

if the phy page is in the swap cache, then private field will contain the swap cache entry for that page.
this entry can be used as the swap pte for installation in the page table.

vma private is used sometimes for containing non-linear vma information.