Re: Driver-driven paging?

[Nick Piggin <nickpiggin@yahoo.com.au>]

Thomas Hellstrom wrote:
> Nick Piggin wrote:

>> If you need for the driver to *then* export these pages out to be mapped
>> by other processes in userspace, I think you run into problems if trying
>> to use nopage. You'll need to go the nopfn route (and thus your mappings
>> must disallow PROT_WRITE && MAP_PRIVATE).
>>
>> But I think that might just work?
>>
> Yes, possibly. What kind of problems would I expect if using nopage? Is 
> it, in particular, legal for a process to call get_user_pages() with the 
> tsk and mm arguments of another process?

Oh that is legal. What I'm thinking you'd have problems with is one
process having its pages imported to the kernel via get_user_pages,
then exported again via an mmap()able device node.

If another process mmaps these pages, you could easily get various
problems like PageAnon being set for a file backed page, or rmap
structures set up incorrectly for the page. It has been a while
since I tried to look at the details, but I would just steer clear
of that case.

Using a nopfn handler (instead of nopage) means that the kernel will
not look at the backing pages at all.

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