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Anyone noticed that there are a lot of cache flushes after kunmap/kunmap

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Subject: Anyone noticed that there are a lot of cache flushes after kunmap/kunmap_atomic is called?
From: David VomLehn <>
Date: Wed, 06 Aug 2008 19:09:57 -0700
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On the MIPS processor, cache flushing is done based on virtual addresses. However, in the Linux kernel, there are a lot of places where memory is mapped with kmap or kmap_atomic, then unmapped with the corresponding kunmap or kunmap_atomic and only *then* is the cache flushed. In other words, we only flush the cache after we have dropped the mapping of memory into a virtual address. I think this is generally wrong.

This may really only affect those of us who have enabled high memory, but it's pretty prevalent in kernel code. We noted this before, but have apparently just been bitten by it. Is it just me or is there a fairly widespread problem for processors that flush the cache using virtual addresses?

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