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Fix a subtraction overflow in get_free_pages. #1261

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Merged
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Jan 14, 2025
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Fix a subtraction overflow in get_free_pages. #1261

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8 changes: 7 additions & 1 deletion src/plan/global.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -266,6 +266,7 @@ pub trait Plan: 'static + HasSpaces + Sync + Downcast {
// the reserved pages is larger than total pages after the copying GC (the reserved pages after a GC
// may be larger than the reserved pages before a GC, as we may end up using more memory for thread local
// buffers for copy allocators).
// 3. the binding disabled GC, and we end up over-allocating beyond the total pages determined by the GC trigger.
let available_pages = total_pages.saturating_sub(reserved_pages);
trace!(
"Total pages = {}, reserved pages = {}, available pages = {}",
Expand All @@ -289,7 +290,12 @@ pub trait Plan: 'static + HasSpaces + Sync + Downcast {
/// Get the number of pages that are NOT used. This is clearly different from available pages.
/// Free pages are unused, but some of them may have been reserved for some reason.
fn get_free_pages(&self) -> usize {
self.get_total_pages() - self.get_used_pages()
let total_pages = self.get_total_pages();
let used_pages = self.get_used_pages();

// It is possible that the used pages is larger than the total pages, so we use saturating
// subtraction. See the comments in `get_available_pages`.
total_pages.saturating_sub(used_pages)
}

/// Return whether last GC was an exhaustive attempt to collect the heap.
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