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only clear RC_USED_NOT_FREE with freelist locked

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This commit is contained in:
4lDO2
2025-11-13 20:47:49 +01:00
parent 96552163f6
commit 33dbf2458e
1 changed files with 31 additions and 31 deletions
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src/memory/mod.rs
+31 -31
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@@ -145,6 +145,11 @@ pub fn allocate_p2frame_complex(
pub unsafe fn deallocate_p2frame(orig_frame: Frame, order: u32) {
let mut freelist = FREELIST.lock();
let initial_info = get_page_info(orig_frame)
.unwrap_or_else(|| panic!("missing PageInfo for {orig_frame:?} being freed"));
initial_info.refcount.store(0, Ordering::Relaxed);
let mut largest_order = order;
let mut current = orig_frame;
@@ -407,7 +412,9 @@ pub struct PageInfo {
/// The RC_USED_NOT_FREE bit marks whether the frame is considered "used" (allocated) or "free"
/// (available to allocator). It must crucially be correct at all times, as the allocator can
/// look at the bits for siblings when determining whether to merge free p2frames with it to
/// form larger p2frames.
/// form larger p2frames. Changing RC_USED_NOT_FREE is only allowed if the freelist is locked,
/// so that the allocator does not simultaneously free a page causing this page to be a sibling
/// to be merged, with potentially unset prev/next.
///
/// If the frame is marked "used", the rest of the bits will track reference count and sharing
/// mode, where refcount is the number of present page table entries that point to this
@@ -814,39 +821,31 @@ impl PageInfo {
match self.refcount() {
None => panic!("refcount was already zero when calling remove_ref!"),
Some(RefCount::One) => {
// Used to be RC_USED_NOT_FREE | ?RC_SHARED_NOT_COW | 1, now becomes 0
self.refcount.store(0, Ordering::Relaxed);
// Used to be RC_USED_NOT_FREE | ?RC_SHARED_NOT_COW | 1, now becomes
// RC_USED_NOT_FREE
self.refcount.store(RC_USED_NOT_FREE, Ordering::Relaxed);
None
}
Some(RefCount::Cow(_) | RefCount::Shared(_)) => {
let new_parsed_value = RefCount::from_raw({
// Used to be RC_USED_NOT_FREE | ?RC_SHARED_NOW_COW | n, now becomes
// RC_USED_NOT_FREE | ?RC_SHARED_NOW_COW | n - 1
Some(RefCount::Cow(_) | RefCount::Shared(_)) => RefCount::from_raw({
// Used to be RC_USED_NOT_FREE | ?RC_SHARED_NOT_COW | n, now becomes
// RC_USED_NOT_FREE | ?RC_SHARED_NOT_COW | n - 1
// if the value returned from fetch_sub indicates count==1, the caller is
// responsible for freeing (return value will be None as mentioned above)
let new_value = self.refcount.fetch_sub(1, Ordering::Relaxed) - 1;
assert_ne!(
new_value,
RC_USED_NOT_FREE - 1,
"refcount underflow, allocator will break"
);
assert_eq!(
new_value & RC_USED_NOT_FREE,
RC_USED_NOT_FREE,
"other malformed refcount"
);
new_value
});
if new_parsed_value.is_none() {
// We were the competing thread that decreased the refcount to zero. That means
// we are now responsible for freeing it, but first we should clear
// RC_USED_NOT_FREE.
self.refcount.store(0, Ordering::Relaxed);
}
new_parsed_value
}
// if the value returned from fetch_sub indicates count==1, the caller is
// responsible for freeing (return value will be None as mentioned above)
let new_value = self.refcount.fetch_sub(1, Ordering::Relaxed) - 1;
assert_ne!(
new_value,
RC_USED_NOT_FREE - 1,
"refcount underflow, allocator will break"
);
assert_eq!(
new_value & RC_USED_NOT_FREE,
RC_USED_NOT_FREE,
"other malformed refcount"
);
new_value
}),
}
}
#[track_caller]
@@ -975,7 +974,8 @@ fn get_free_alloc_page_info(frame: Frame) -> PageInfoFree<'static> {
let i = get_page_info(frame).unwrap_or_else(|| {
panic!("allocator-owned frames need a PageInfo, but none for {frame:?}")
});
i.as_free().unwrap() //.unwrap_or_else(|| panic!("expected frame to be free, but {frame:?} wasn't, in {i:?}"))
i.as_free()
.unwrap_or_else(|| panic!("expected frame to be free, but {frame:?} wasn't, in {i:?}"))
}
pub struct Segv;