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Fix RawMemoryFreeList panicking when it grows up to its limit #1627

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Fix RawMemoryFreeList panicking when it grows up to its limit #1627

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9c900f0
Fix grow extent underflow in RawMemoryFreeList::raise_high_water
Ran-Mewo Nov 3, 2026
0e49cd7
Count the partial last block in RawMemoryFreeList::current_capacity
Ran-Mewo Nov 3, 2026
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38 changes: 31 additions & 7 deletions src/util/raw_memory_freelist.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -94,9 +94,6 @@ impl RawMemoryFreeList {
fn units_per_block(&self) -> i32 {
(conversions::pages_to_bytes(self.pages_per_block as _) >> LOG_BYTES_IN_UNIT) as _
}
fn units_in_first_block(&self) -> i32 {
self.units_per_block() - self.heads - 1
}
pub fn default_block_size(units: i32, heads: i32) -> i32 {
usize::min(Self::size_in_pages(units, heads) as _, 16) as _
}
Expand Down Expand Up @@ -136,9 +133,8 @@ impl RawMemoryFreeList {
}

fn current_capacity(&self) -> i32 {
let list_blocks = conversions::bytes_to_pages_up(self.high_water - self.base) as i32
/ self.pages_per_block;
self.units_in_first_block() + (list_blocks - 1) * self.units_per_block()
let list_units = ((self.high_water - self.base) >> LOG_BYTES_IN_UNIT) as i32;
list_units - self.heads - 1
}

pub fn grow_freelist(&mut self, units: i32) -> bool {
Expand Down Expand Up @@ -216,7 +212,7 @@ impl RawMemoryFreeList {
"Attempt to grow FreeList beyond limit"
);
if self.high_water + grow_extent > self.limit {
grow_extent = self.high_water - self.limit;

@wks wks Nov 4, 2026

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It looks like the code in the Rust MMTk core is ported from this piece of code in JikesRVM:

  /**
   * Raise the high water mark by requesting more pages from the OS
   *
   * @param blocks block count to raise by
   */
  private void raiseHighWater(int blocks) {
    Extent growExtent = Conversions.pagesToBytes(pagesPerBlock * blocks);
    if (highWater.EQ(limit)) {
      Log.write("limit=", limit);
      Log.write(", highWater=", highWater);
      Log.writeln(", growExtent=", growExtent);
      VM.assertions.fail("Attempt to grow FreeList beyond limit");
    }
    if (highWater.plus(growExtent).GT(limit)) {
      growExtent = highWater.diff(limit).toWord().toExtent();
    }
    mmap(highWater, growExtent);
    highWater = highWater.plus(growExtent);
  }

highWater.diff(limit) means highWater - limit. The highWater should not be higher than than limit, and the result of the diff shall be negative. It is a bug in JikesRVM, and I am surprised it has existed since 2016 and never gets found.

grow_extent = self.limit - self.high_water;
}
self.mmap(self.high_water, grow_extent);
self.high_water += grow_extent;
Expand Down Expand Up @@ -585,4 +581,32 @@ mod tests {
let res4 = l.alloc(1);
assert_eq!(res4, 4);
}

#[test]
fn grow_into_partial_last_block() {
let _guard = match MUTEX.lock() {
Ok(guard) => guard,
Err(poisoned) => poisoned.into_inner(),
};
let start = crate::util::test_util::RAW_MEMORY_FREELIST_TEST_REGION.start;
let units = 12264;
let grain = 1024;
let pages = RawMemoryFreeList::size_in_pages(units, 1);
let pages_per_block = RawMemoryFreeList::default_block_size(units, 1);
// The limit falls in the middle of a block, so the last growth is clamped.
assert_ne!(pages % pages_per_block, 0);
let mut l = RawMemoryFreeList::new(
start,
start + conversions::pages_to_bytes(pages as _),
pages_per_block,
units,
grain,
1,
MmapStrategy::TEST,
);
for _ in 0..units / grain {
assert!(l.grow_freelist(grain));
}
assert!(!l.grow_freelist(grain));
}
}
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