Re: [RFC V2 11/12] mm: Tag VMA with VM_CDM flag during page fault

From: Dave Hansen
Date: Tue Jan 31 2017 - 13:00:25 EST

On 01/30/2017 09:10 PM, Anshuman Khandual wrote:
>> This is happening with mmap_sem held for read. Correct? Is it OK that
>> you're modifying the VMA? That vm_flags manipulation is non-atomic, so
>> how can that even be safe?
> Hmm. should it be done with mmap_sem being held for write. Will look
> into this further. But intercepting the page faults inside alloc_pages_vma()
> for tagging the VMA is okay from over all design perspective ?. Or this
> should be moved up or down the call chain in the page fault path ?

Doing it in the fault path seems wrong to me.

Apps have to take *explicit* action to go and get access to device
memory. It seems like we should mark the VMA *then*, at the time of the
explicit action. I also think _implying_ that we want KSM, etc...
turned off just because of the target of an mbind() is a bad idea. Apps
have to ask for this stuff *explicitly*, so why not also have them turn
KSM off explicitly?