On 22.06.23 09:27, Vivek Kasireddy wrote:
The first patch ensures that the mappings needed for handling mmap operation would be managed by using the pfn instead of struct page. The second patch restores support for mapping hugetlb pages where subpages of a hugepage are not directly used anymore (main reason for revert) and instead the hugetlb pages and the relevant offsets are used to populate the scatterlist for dma-buf export and for mmap operation. Testcase: default_hugepagesz=2M hugepagesz=2M hugepages=2500 options were passed to the Host kernel and Qemu was launched with these relevant options: qemu-system-x86_64 -m 4096m.... -device virtio-gpu-pci,max_outputs=1,blob=true,xres=1920,yres=1080 -display gtk,gl=on -object memory-backend-memfd,hugetlb=on,id=mem1,size=4096M -machine memory-backend=mem1 Replacing -display gtk,gl=on with -display gtk,gl=off above would exercise the mmap handler.
While I think the VM_PFNMAP approach is much better and should fix that issue at hand, I thought more about missing memlock support and realized that we might have to fix something else. SO I'm going to raise the issue here.
I think udmabuf chose the wrong interface to do what it's doing, that makes it harder to fix it eventually.
Instead of accepting a range in a memfd, it should just have accepted a user space address range and then used pin_user_pages(FOLL_WRITE|FOLL_LONGTERM) to longterm-pin the pages "officially".
So what's the issue? Udma effectively pins pages longterm ("possibly forever") simply by grabbing a reference on them. These pages might easily reside in ZONE_MOVABLE or in MIGRATE_CMA pageblocks.
So what udmabuf does is break memory hotunplug and CMA, because it turns pages that have to remain movable unmovable.
In the pin_user_pages(FOLL_LONGTERM) case we make sure to migrate these pages. See mm/gup.c:check_and_migrate_movable_pages() and especially folio_is_longterm_pinnable(). We'd probably have to implement something similar for udmabuf, where we detect such unpinnable pages and migrate them.
For example, pairing udmabuf with vfio (which pins pages using pin_user_pages(FOLL_LONGTERM)) in QEMU will most probably not work in all cases: if udmabuf longterm pinned the pages "the wrong way", vfio will fail to migrate them during FOLL_LONGTERM and consequently fail pin_user_pages(). As long as udmabuf holds a reference on these pages, that will never succeed.
There are *probably* more issues on the QEMU side when udmabuf is paired with things like MADV_DONTNEED/FALLOC_FL_PUNCH_HOLE used for virtio-balloon, virtio-mem, postcopy live migration, ... for example, in the vfio/vdpa case we make sure that we disallow most of these, because otherwise there can be an accidental "disconnect" between the pages mapped into the VM (guest view) and the pages mapped into the IOMMU (device view), for example, after a reboot.
-- Cheers, David / dhildenb
