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

[David Hildenbrand <david@redhat.com>]

On 15.03.22 18:43, Matthew Wilcox wrote:
> On Tue, Mar 15, 2022 at 04:45:13PM +0100, 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@mediatek.com> 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.
> 
> We could add an assertion ... I just tried this:
> 
> +++ b/include/linux/page-flags.h
> @@ -342,14 +342,16 @@ static __always_inline                                                    \
>  void __folio_set_##lname(struct folio *folio)                          \
>  { __set_bit(PG_##lname, folio_flags(folio, FOLIO_##policy)); }         \
>  static __always_inline void __SetPage##uname(struct page *page)                \
> -{ __set_bit(PG_##lname, &policy(page, 1)->flags); }
> +{ VM_BUG_ON_PGFLAGS(atomic_read(&policy(page, 1)->_refcount), page);   \
> +  __set_bit(PG_##lname, &policy(page, 1)->flags); }
> 
>  #define __CLEARPAGEFLAG(uname, lname, policy)                          \
>  static __always_inline                                                 \
>  void __folio_clear_##lname(struct folio *folio)                                \
>  { __clear_bit(PG_##lname, folio_flags(folio, FOLIO_##policy)); }       \
>  static __always_inline void __ClearPage##uname(struct page *page)      \
> -{ __clear_bit(PG_##lname, &policy(page, 1)->flags); }
> +{ VM_BUG_ON_PGFLAGS(atomic_read(&policy(page, 1)->_refcount), page);   \
> +  __clear_bit(PG_##lname, &policy(page, 1)->flags); }
> 
>  #define TESTSETFLAG(uname, lname, policy)                              \
>  static __always_inline                                                 \
> 
> ... but it dies _really_ early:
> 
> (gdb) bt
> #0  0xffffffff820055e5 in native_halt ()
>     at ../arch/x86/include/asm/irqflags.h:57
> #1  halt () at ../arch/x86/include/asm/irqflags.h:98
> #2  early_fixup_exception (regs=regs@entry=0xffffffff81e03cf8,
>     trapnr=trapnr@entry=6) at ../arch/x86/mm/extable.c:283
> #3  0xffffffff81ff243c in do_early_exception (regs=0xffffffff81e03cf8,
>     trapnr=6) at ../arch/x86/kernel/head64.c:419
> #4  0xffffffff81ff214f in early_idt_handler_common ()
>     at ../arch/x86/kernel/head_64.S:417
> #5  0x0000000000000000 in ?? ()
> 
> and honestly, I'm not sure how to debug something that goes wrong this
> early.  Maybe I need to make that start warning 5 seconds after boot
> or only if we're not in pid 1, or something ...

Maybe checking for "system_state >= SYSTEM_RUNNING" or "system_state >=
SYSTEM_SCHEDULING" to exclude early boot where no (real) concurrency is
happening. But I assume you'll still get plenty of such reports.

-- 
Thanks,

David / dhildenb