removing people that are probably not interested from CC adding dri-devel

On 9/11/19 11:08 AM, Koenig, Christian wrote:

Am 10.09.19 um 21:26 schrieb Thomas Hellström (VMware):

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On 9/10/19 6:11 PM, Andy Lutomirski wrote:

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On Sep 5, 2019, at 8:24 AM, Christoph Hellwig hch@infradead.org wrote:

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On Thu, Sep 05, 2019 at 05:21:24PM +0200, Thomas Hellström (VMware) wrote:

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On 9/5/19 4:15 PM, Dave Hansen wrote: Hi Thomas,

Thanks for the second batch of patches!  These look much improved on all fronts.

Yes, although the TTM functionality isn't in yet. Hopefully we won't have to bother you with those though, since this assumes TTM will be using the dma API.

Please take a look at dma_mmap_prepare and dma_mmap_fault in this branch:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-mmap-imp...

they should allow to fault dma api pages in the page fault handler. But this is totally hot off the press and not actually tested for the last few patches.  Note that I've also included your two patches from this series to handle SEV.

I read that patch, and it seems like you’ve built in the assumption that all pages in the mapping use identical protection or, if not, that the same fake vma hack that TTM already has is used to fudge around it.  Could it be reworked slightly to avoid this?

I wonder if it’s a mistake to put the encryption bits in vm_page_prot at all.

From my POW, the encryption bits behave quite similar in behaviour to the caching mode bits, and they're also in vm_page_prot. They're the reason TTM needs to modify the page protection in the fault handler in the first place.

The problem seen in TTM is that we want to be able to change the vm_page_prot from the fault handler, but it's problematic since we have the mmap_sem typically only in read mode. Hence the fake vma hack. From what I can tell it's reasonably well-behaved, since pte_modify() skips the bits TTM updates, so mprotect() and mremap() works OK. I think split_huge_pmd may run into trouble, but we don't support it (yet) with TTM.

Ah! I actually ran into this while implementing huge page support for TTM and never figured out why that doesn't work. Dropped CPU huge page support because of this.

By incident, I got slightly sidetracked the other day and started looking at this as well. Got to the point where I figured out all the hairy alignment issues and actually got huge_fault() calls, but never implemented the handler. I think that's definitely something worth having. Not sure it will work for IO memory, though, (split_huge_pmd will just skip non-page-backed memory) but if we only support VM_SHARED (non COW) vmas there's no reason to split the huge pmds anyway. Definitely something we should have IMO.

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We could probably get away with a WRITE_ONCE() update of the vm_page_prot before taking the page table lock since

a) We're locking out all other writers. b) We can't race with another fault to the same vma since we hold an address space lock ("buffer object reservation") c) When we need to update there are no valid page table entries in the vma, since it only happens directly after mmap(), or after an unmap_mapping_range() with the same address space lock. When another reader (for example split_huge_pmd()) sees a valid page table entry, it also sees the new page protection and things are fine.

Yeah, that's exactly why I always wondered why we need this hack with the vma copy on the stack.

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But that would really be a special case. To solve this properly we'd probably need an additional lock to protect the vm_flags and vm_page_prot, taken after mmap_sem and i_mmap_lock.

Well we already have a special lock for this: The reservation object. So memory barriers etc should be in place and I also think we can just update the vm_page_prot on the fly.

I agree. This is needed for huge pages. We should make this change, and perhaps add the justification above as a comment.

/Thomas

Christian.

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/Thomas