Fix a subtraction overflow in get_free_pages. #1261
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The used pages can also be greater than the total pages for the same reason as those in computing `get_available_pages`, and it can also happen if the VM binding disabled GC, in which case we may over-allocate without triggering GC. When it overflows, `get_free_pages` will cause subtraction overflow, and will panic in debug build. We switch to `saturating_sub` so that it will return 0 if overflow happens. It still makes sense. 0 means there is no free pages because we are over-allocating beyond the current heap size set by the GC trigger.
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Rust 1.84 added a new lint unnecessary_map_or, and the style check failed for that reason. I'll fix that separately. |
peterzhu2118
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mmtk/mmtk-core#1261 fixes an issue where the following script causes a Rust panic: GC.disable 10_000.times { Object.new } puts GC.stat
matzbot
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mmtk/mmtk-core#1261 fixes an issue where the following script causes a Rust panic: GC.disable 10_000.times { Object.new } puts GC.stat ruby/mmtk@6191ee994a
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The used pages can also be greater than the total pages for the same reason as those in computing
get_available_pages, and it can also happen if the VM binding disabled GC, in which case we may over-allocate without triggering GC. When it overflows,get_free_pageswill cause subtraction overflow, and will panic in debug build.We switch to
saturating_subso that it will return 0 if overflow happens. It still makes sense. 0 means there is no free pages because we are over-allocating beyond the current heap size set by the GC trigger.