gh-145615: Fix mimalloc page leak in the free-threaded build#145626
Merged
colesbury merged 4 commits intopython:mainfrom Mar 9, 2026
Merged
gh-145615: Fix mimalloc page leak in the free-threaded build#145626colesbury merged 4 commits intopython:mainfrom
colesbury merged 4 commits intopython:mainfrom
Conversation
Fix three issues that caused mimalloc pages to be leaked until the owning thread exited: 1. In _PyMem_mi_page_maybe_free(), move pages out of the full queue when relying on QSBR to defer freeing the page. Pages in the full queue are never searched by mi_page_queue_find_free_ex(), so a page left there is unusable for allocations. 2. Move _PyMem_mi_page_clear_qsbr() from _mi_page_free_collect() to _mi_page_thread_free_collect() where it only fires when all blocks on the page are free (used == 0). The previous placement was too broad: it cleared QSBR state whenever local_free was non-NULL, but _mi_page_free_collect() is called from non-allocation paths (e.g., page visiting in mi_heap_visit_blocks) where the page is not being reused. 3. In _PyMem_mi_page_maybe_free(), use the page's heap tld to find the correct thread state for QSBR list insertion instead of PyThreadState_GET(). During stop-the-world pauses, the function may process pages belonging to other threads, so the current thread state is not necessarily the owner of the page.
colesbury
added
topic-free-threading
needs backport to 3.14
colesbury
added
the
🔨 test-with-buildbots
This comment was marked as outdated.
This comment was marked as outdated.
bedevere-bot
removed
the
🔨 test-with-buildbots
colesbury
added
the
🔨 test-with-buildbots
|
🤖 New build scheduled with the buildbot fleet by @colesbury for commit 2516a1a 🤖 Results will be shown at: https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F145626%2Fmerge If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again. |
bedevere-bot
removed
the
🔨 test-with-buildbots
Contributor
|
The changes look good to me, though I am not intimately familiar with mimalloc or our modifications to it. The buildbot failure seems like it might be real though. It looks like |
Contributor
Author
|
I don't think the buildbot failure is related. The PR only touches mimalloc and |
mpage
approved these changes
Mar 9, 2026
|
Thanks @colesbury for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14. |
miss-islington
pushed a commit
to miss-islington/cpython
that referenced
this pull request
Mar 9, 2026
…ythongh-145626) Fix three issues that caused mimalloc pages to be leaked until the owning thread exited: 1. In _PyMem_mi_page_maybe_free(), move pages out of the full queue when relying on QSBR to defer freeing the page. Pages in the full queue are never searched by mi_page_queue_find_free_ex(), so a page left there is unusable for allocations. 2. Move _PyMem_mi_page_clear_qsbr() from _mi_page_free_collect() to _mi_page_thread_free_collect() where it only fires when all blocks on the page are free (used == 0). The previous placement was too broad: it cleared QSBR state whenever local_free was non-NULL, but _mi_page_free_collect() is called from non-allocation paths (e.g., page visiting in mi_heap_visit_blocks) where the page is not being reused. 3. In _PyMem_mi_page_maybe_free(), use the page's heap tld to find the correct thread state for QSBR list insertion instead of PyThreadState_GET(). During stop-the-world pauses, the function may process pages belonging to other threads, so the current thread state is not necessarily the owner of the page. (cherry picked from commit d76df75f51e662fd15ebe00e107058841de94860) Co-authored-by: Sam Gross <colesbury@gmail.com>
|
GH-145691 is a backport of this pull request to the 3.14 branch. |
colesbury
added a commit
that referenced
this pull request
Mar 9, 2026
…h-145626) (#145691) Fix three issues that caused mimalloc pages to be leaked until the owning thread exited: 1. In _PyMem_mi_page_maybe_free(), move pages out of the full queue when relying on QSBR to defer freeing the page. Pages in the full queue are never searched by mi_page_queue_find_free_ex(), so a page left there is unusable for allocations. 2. Move _PyMem_mi_page_clear_qsbr() from _mi_page_free_collect() to _mi_page_thread_free_collect() where it only fires when all blocks on the page are free (used == 0). The previous placement was too broad: it cleared QSBR state whenever local_free was non-NULL, but _mi_page_free_collect() is called from non-allocation paths (e.g., page visiting in mi_heap_visit_blocks) where the page is not being reused. 3. In _PyMem_mi_page_maybe_free(), use the page's heap tld to find the correct thread state for QSBR list insertion instead of PyThreadState_GET(). During stop-the-world pauses, the function may process pages belonging to other threads, so the current thread state is not necessarily the owner of the page. (cherry picked from commit d76df75) Co-authored-by: Sam Gross <colesbury@gmail.com>
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
3 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Fix three issues that caused mimalloc pages to be leaked until the owning thread exited:
In
_PyMem_mi_page_maybe_free(), move pages out of the full queue when relying on QSBR to defer freeing the page. Pages in the full queue are never searched bymi_page_queue_find_free_ex(), so a page left there is unusable for allocations.Move
_PyMem_mi_page_clear_qsbr()from_mi_page_free_collect()to_mi_page_thread_free_collect()where it only fires when all blocks on the page are free (used == 0). The previous placement was too broad: it cleared QSBR state whenever local_free was non-NULL, but_mi_page_free_collect()is called from non-allocation paths (e.g., page visiting inmi_heap_visit_blocks) where the page is not being reused.In
_PyMem_mi_page_maybe_free(), use the page's heap tld to find the correct thread state for QSBR list insertion instead ofPyThreadState_GET(). During stop-the-world pauses, the function may process pages belonging to other threads, so the current thread state is not necessarily the owner of the page.