On 15.03.22 05:21, Andrew Morton wrote: > On Tue, 15 Mar 2022 11:05:15 +0800 Andrew Yang <andrew.yang@xxxxxxxxxxxx> wrote: > >> When memory is tight, system may start to compact memory for large >> continuous memory demands. If one process tries to lock a memory page >> that is being locked and isolated for compaction, it may wait a long time >> or even forever. This is because compaction will perform non-atomic >> PG_Isolated clear while holding page lock, this may overwrite PG_waiters >> set by the process that can't obtain the page lock and add itself to the >> waiting queue to wait for the lock to be unlocked. >> >> CPU1 CPU2 >> lock_page(page); (successful) >> lock_page(); (failed) >> __ClearPageIsolated(page); SetPageWaiters(page) (may be overwritten) >> unlock_page(page); >> >> The solution is to not perform non-atomic operation on page flags while >> holding page lock. > > Sure, the non-atomic bitop optimization is really risky and I suspect > we reach for it too often. Or at least without really clearly > demonstrating that it is safe, and documenting our assumptions. I agree. IIRC, non-atomic variants are mostly only safe while the refcount is 0. Everything else is just absolutely fragile. -- Thanks, David / dhildenb
