demand paging redux #402
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demand paging redux #402
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corrects comments removes dead code makes page and offset calculations more consistent
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This PR is a forward-port / rewrite of the demand-paging implementation from v1.14 (see urbit/urbit#6063, urbit/urbit#6126, urbit/urbit#6127, and urbit/urbit#6152). The original scope has been decreased, and the implementation simplified: i/o errors are not retried, the dirty page bitmap is manipulated with much simpler code, page offsets/pointers are calculated with macros, &c. There are additional layers of snapshot validation for updates (controlled at compile time, always on as of this PR); clean pages are compared to disk both before and after update. (This validation should stay on for pre-release testing, and possibly for initial release as well.)
This PR has been tested extensively on live ships; the corruption issues that plagued v1.14 cannot be reproduced.
Fixes #188. Supersedes #277. (Was previously opened as #401, but a typo in the branch name was preventing updates.)