Changelog
================================================================
v2: Use mapcount instead of refcount
Synchronized with IOMMU Observability changes.
================================================================
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->mapcount is used in order to track number of entries at each page
table.
Microbenchmark data using iova_stress[1]:
Base:
$ ./iova_stress -s 16
dma_size: 4K iova space: 16T iommu: ~ 32847M time: 36.074s
Fix:
$ ./iova_stress -s 16
dma_size: 4K iova space: 16T iommu: ~ 27M time: 38.870s
The test maps/unmaps 4K pages and cycles through the IOVA space in a tight loop.
Base uses 32G of memory, and test completes in 36.074s
Fix uses 0G of memory, and test completes in 38.870s.
I believe the proposed fix is a good compromise 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->_mapcount 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 | 154 ++++++++++++++++++++++++++++--------
drivers/iommu/intel/iommu.h | 42 ++++++++--
drivers/iommu/iommu-pages.h | 30 +++++--
3 files changed, 180 insertions(+), 46 deletions(-)
--
2.44.0.769.g3c40516874-goog
