Benjamin Herrenschmidt wrote:
OK. i was reffering to another approach: Copying _to_ VRAM /AGP: lock_mmap_sems() unmap_mapping_range() (or similar) copy() / flip() foreach_affected_vma{ io_remap_pfn_range() /* Map vram / AGP space */ } unlock_mmap_sem() This works like a charm in the drm memory manager but it requires the lock of the mmap sems from all affected processes, and the locking order must be the same all the time otherwise deadlocks will occur. do_no_page() is smart enough to recheck the pte when it retakes the page table spinlock(), so if the pte has been populated by someone while in the driver nopage(), the returned struct page will simply be discarded.Now, of course, the real problem is that we don't have struct page for vram.... There are two ways out of this: - Enforce use of sparsemem and create struct page for vram. That will probably make a few people jump out of their seats in x86 land but that's what we do for cell and SPUs for now. - There's a prooposal that I'm pusing to add a way for no_page() to return a NOPAGE_RETRY error, which essentially causes it to go all the way back to userland and re-do the access. I want that to be able to handle signals while blocked inside no_page() but that could -also- be used to have no_page() setup the PTE mappings itself and return NOPAGE_RETRY, thus avoiding the need for a struct page. Now I do not -ever- want to see drivers mucking around with PTEs directly, however, we can provide something in mm/memory.c that a driver can call from within no_page() to perform the set_pte() along with all the necessary locking, flushes, etc... The base code for NOPAGE_RETRY should get in 2.6.19 soon (one of these days). io_remap_pfn_range() should do the job of setting up the new ptes, but it needs the mmap_sem, so if that one is held while blocked in nopage(), a deadlock will occur. Here, the NOPAGE_RETRY will obviously do the job. When io_remap_pfn_range() has finished setting up the ptes, one can simply return a bogus page to nopage() if it insists on retrying. Until NOPAGE_RETRY is implemented, I'm afraid I'm stuck with the approach outlined above. /Thomas Ben. |
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