Hello,
I'm trying to understand how TTM buffer object mapping works on Linux, to make this behave properly on FreeBSD.
Here's what I think I understand:
When a buffer object is mmap()'d, ttm_bo_vm_open() is called. When there's a page fault, the page is looked up and inserted in the VMA using vm_insert_mixed(). When a buffer object is munmap()'d, ttm_bo_vm_close() is called, which drops a reference. When the last reference is dropped, the buffer object is destroyed.
What's still not clear to me is how munmap() works here. After talking about this on IRC with some people, I think that unmap_mapping_range() (called by ttm_bo_unmap_virtual_locked()) is equivalent to calling munmap() from userland. Is that true?
When a buffer object is moved, what happens to the mapping?
In particular, I see in ttm_bo_move_accel_cleanup() that the ttm structure can be transferred to ghost_obj, which is destroyed shortly after. This ends up in ttm_put_pages() which uses __free_page(), for each page of the buffer object. At this stage, is the ghost object already munmap()'d? Or does __free_page() unmap a page implicitly (ie. remove it from VMA)?
Sorry if my questions are stupid, I'm rather new to memory management.