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[35.194.87.55]) by smtp.gmail.com with ESMTPSA id m18-20020a05620a221200b0077d85695db4sm371893qkh.99.2023.12.20.19.19.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 20 Dec 2023 19:19:18 -0800 (PST) From: Pasha Tatashin To: akpm@linux-foundation.org, linux-mm@kvack.org, pasha.tatashin@soleen.com, linux-kernel@vger.kernel.org, rientjes@google.com, dwmw2@infradead.org, baolu.lu@linux.intel.com, joro@8bytes.org, will@kernel.org, robin.murphy@arm.com, iommu@lists.linux.dev Subject: [RFC 0/3] iommu/intel: Free empty page tables on unmaps Date: Thu, 21 Dec 2023 03:19:12 +0000 Message-ID: <20231221031915.619337-1-pasha.tatashin@soleen.com> X-Mailer: git-send-email 2.43.0.472.g3155946c3a-goog MIME-Version: 1.0 X-Rspamd-Queue-Id: 6CFA08001B X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: umpafhkxkujmwru7rhefbbiw1a18qfxg X-HE-Tag: 1703128759-503695 X-HE-Meta: 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 eGld5Tkm kKKWL1JiL/cM7rI/SWlMy8c3V0sIqkJg3ccVLiUKXbeB9/OVpwW8upbvCQSf+DX+oDrjSQznRhypomzW73rE2Xuso7svSNd81q6sQIBEmb+xARH1GLaOM5zWjO8NAUyqNUcGkJQNeRM9TL6YzeBlnltmArlxYa1xaQVWzPEWtg2y9kPb3NmSOtboQJXNHCmJsNpYJBjs3XmE1CQC6g5FARlWe6Ok/jwE/u1VC4bETFZYrvJd6mTee28LGIbO0wjZAO2gCmrVpbCPAm4vl2SrGwUdNjgsGf0KL9P/mxPCVZwQWtBihXS+/Sp445JjxP6S4rALme6G3gmnsboqOgvTAQofpUg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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(-)