Re: [PATCH] udmabuf: revert 'Add support for mapping hugepages (v4)'

[David Hildenbrand]

On 12.06.23 09:10, Kasireddy, Vivek wrote:
> Hi Mike,

Hi Vivek,

> Sorry for the late reply; I just got back from vacation.
> If it is unsafe to directly use the subpages of a hugetlb page, then reverting
> this patch seems like the only option for addressing this issue immediately.
> So, this patch is
> Acked-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
>
> As far as the use-case is concerned, there are two main users of the udmabuf
> driver: Qemu and CrosVM VMMs. However, it appears Qemu is the only one
> that uses hugetlb pages (when hugetlb=on is set) as the backing store for
> Guest (Linux, Android and Windows) system memory. The main goal is to
> share the pages associated with the Guest allocated framebuffer (FB) with
> the Host GPU driver and other components in a zero-copy way. To that end,
> the guest GPU driver (virtio-gpu) allocates 4k size pages (associated with
> the FB) and pins them before sharing the (guest) physical (or dma) addresses
> (and lengths) with Qemu. Qemu then translates the addresses into file
> offsets and shares these offsets with udmabuf.

Is my understanding correct, that we can effectively long-term pin 
(worse than mlock) 64 MiB per UDMABUF_CREATE, allowing eventually !root 
users

ll /dev/udmabuf
crw-rw---- 1 root kvm 10, 125 12. Jun 08:12 /dev/udmabuf

to bypass there effective MEMLOCK limit, fragmenting physical memory and 
breaking swap?


Regarding the udmabuf_vm_fault(), I assume we're mapping pages we 
obtained from the memfd ourselves into a special VMA (mmap() of the 
udmabuf). I'm not sure how well shmem pages are prepared for getting 
mapped by someone else into an arbitrary VMA (page->index?).

... also, just imagine someone doing FALLOC_FL_PUNCH_HOLE / ftruncate() 
on the memfd. What's mapped into the memfd no longer corresponds to 
what's pinned / mapped into the VMA.


Was linux-mm (and especially shmem maintainers, ccing Hugh) involved in 
the upstreaming of udmabuf?

--
Cheers,

David / dhildenb