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u3: document snapshot system invariants

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Joe Bryan 2022-12-02 09:57:29 -05:00
parent d94d6d3dd2
commit fee50375c9
1 changed files with 22 additions and 0 deletions
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pkg/urbit/noun/events.c
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@ -50,6 +50,28 @@
//! - memory protections (and file-backed mappings) are re-established.
//! - patch files are deleted.
//!
//! ### invariants
//!
//! definitions:
//! - a clean page is PROT_READ and 0 in the bitmap
//! - a dirty page is (PROT_READ|PROT_WRITE) and 1 in the bitmap
//! - the guard page is PROT_NONE and 1 in the bitmap (XX assumed)
//!
//! assumptions:
//! - all memory access patterns are outside-in, a page at a time
//! - ad-hoc exceptions are supported by calling u3e_ward()
//!
//! - there is a single guard page, between the segments
//! - dirty pages only become clean by being:
//! - loaded from a snapshot during initialization
//! - present in a snapshot after save
//! - clean pages only become dirty by being:
//! - modified (and caught by the fault handler)
//! - orphaned due to segment truncation (explicitly dirtied)
//! - at points of quiescence (initialization, after save)
//! - all pages of the north and south segments are clean
//! - all other pages are dirty
//!
//! ### limitations
//!
//! - loom page size is fixed (16 KB), and must be a multiple of the