Questions about TTM buffer object maping

[Jerome Glisse]

On Wed, Jul 10, 2013 at 8:27 PM, Jean-Sébastien Pédron
<jean-sebastien.pedron at dumbbell.fr> wrote:
> 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?

Yes that's true.

> When a buffer object is moved, what happens to the mapping?

unmap_mapping_range is call from ttm_bo_move (indirectly through the
helper function).

>
> 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)?

Yes object is unmapped prior to move.

Cheers,
Jerome

>
> Sorry if my questions are stupid, I'm rather new to memory management.
>
> --
> Jean-Sébastien Pédron
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