This series frees empty page tables on unmaps. It intends to be a low overhead feature. The read-writer lock is used to synchronize page table, but most of time the lock is held is reader. It is held as a writer for short period of time when unmapping a page that is bigger than the current iova request. For all other cases this lock is read-only. page->refcount is used in order to track number of entries at each page table. Microbenchmark data using iova_stress[1]: Base: yqbtg12:/home# ./iova_stress -s 16 dma_size: 4K iova space: 16T iommu: ~ 32783M time: 22.297s /iova_stress -s 16 dma_size: 4K iova space: 16T iommu: ~ 0M time: 23.388s The test maps/unmaps 4K pages and cycles through the IOVA space. Base uses 32G of memory, and test completes in 22.3S. Fix uses 0G of memory, and test completes in 23.4s. I believe the proposed fix is a good compromize in terms of complexity/ scalability. A more scalable solution would be to spread read/writer lock per-page table, and user page->private field to store the lock itself. However, since iommu already has some protection: i.e. no-one touches the iova space of the request map/unmap we can avoid the extra complexity and rely on a single per page table RW lock, and be in a reader mode most of the time. [1] https://github.com/soleen/iova_stress Pasha Tatashin (3): iommu/intel: Use page->refcount to count number of entries in IOMMU iommu/intel: synchronize page table map and unmap operations iommu/intel: free empty page tables on unmaps drivers/iommu/intel/iommu.c | 153 ++++++++++++++++++++++++++++-------- drivers/iommu/intel/iommu.h | 44 +++++++++-- 2 files changed, 158 insertions(+), 39 deletions(-) -- 2.43.0.472.g3155946c3a-goog
