Messages in this thread | | | Subject | TTM huge page-faults WAS: Re: [RFC PATCH 1/2] x86: Don't let pgprot_modify() change the page encryption bit | From | Thomas Hellström (VMware) <> | Date | Wed, 11 Sep 2019 12:10:21 +0200 |
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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): >> On 9/10/19 6:11 PM, Andy Lutomirski wrote: >>>> On Sep 5, 2019, at 8:24 AM, Christoph Hellwig <hch@infradead.org> >>>> wrote: >>>> >>>>> On Thu, Sep 05, 2019 at 05:21:24PM +0200, Thomas Hellström (VMware) >>>>> wrote: >>>>>> 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-improvements >>>> >>>> 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.
>> 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. > >> 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. > >> /Thomas >> >> >> >>
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