Re: [PATCH] mm/migrate: fix race between lock page and clear PG_Isolated

[David Hildenbrand]

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