On Tue, 15 Mar 2022, David Hildenbrand wrote: > 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. It is normal and correct to use __SetPageFlag(page) on a page just allocated from the buddy, and not yet logically visible to others: that has refcount 1. Of course, it might have refcount 2 or more, through being speculatively visible to get_page_unless_zero() users: perhaps through earlier usage of the same struct page, or by physical scan of memmap. Those few such others - compaction's isolate_migratepages_block() is the one I know best - must be very careful in their sequence of operations. Preliminary read-only checks are usually okay (but some VM_BUG_ON_PGFLAGS are increasingly problematic: I've had to turn off that CONFIG), then get_page_unless_zero(), then read-only check that the page is of the manageable kind (PageLRU in my world), and only then can it be safe to lock the page - which of course touches page flags, and so would be problematic for a racing user's __SetPageFlag(page). But PageMovable and PageIsolated are beyond my ken: I can't say there. Hugh
