Re: [PATCH hotfix] mm: fix crashes from deferred split racing folio migration

Baolin Wang
2024-07-03T01:51:47Z

On 2024/7/3 00:15, Hugh Dickins wrote:
> On Tue, 2 Jul 2024, Baolin Wang wrote:
>> On 2024/7/2 15:40, Hugh Dickins wrote:
>>> Even on 6.10-rc6, I've been seeing elusive "Bad page state"s (often on
>>> flags when freeing, yet the flags shown are not bad: PG_locked had been
>>> set and cleared??), and VM_BUG_ON_PAGE(page_ref_count(page) == 0)s from
>>> deferred_split_scan()'s folio_put(), and a variety of other BUG and WARN
>>> symptoms implying double free by deferred split and large folio migration.
>>>
>>> 6.7 commit 9bcef5973e31 ("mm: memcg: fix split queue list crash when large
>>> folio migration") was right to fix the memcg-dependent locking broken in
>>> 85ce2c517ade ("memcontrol: only transfer the memcg data for migration"),
>>> but missed a subtlety of deferred_split_scan(): it moves folios to its own
>>> local list to work on them without split_queue_lock, during which time
>>> folio->_deferred_list is not empty, but even the "right" lock does nothing
>>> to secure the folio and the list it is on.
>>>
>>> Fortunately, deferred_split_scan() is careful to use folio_try_get(): so
>>> folio_migrate_mapping() can avoid the race by folio_undo_large_rmappable()
>>> while the old folio's reference count is temporarily frozen to 0 - adding
>>> such a freeze in the !mapping case too (originally, folio lock and
>
> (I should have added "isolation and" into that list of conditions.)
>
>>> unmapping and no swap cache left an anon folio unreachable, so no freezing
>>> was needed there: but the deferred split queue offers a way to reach it).
>>
>> Thanks Hugh.
>>
>> But after reading your analysis, I am concerned that the
>> folio_undo_large_rmappable() and deferred_split_scan() may still encounter a
>> race condition with the local list, even with your patch.
>>
>> Suppose folio A has already been queued into the local list in
>> deferred_split_scan() by thread A, but fails to 'folio_trylock' and then
>> releases the reference count. At the same time, folio A can be frozen by
>> another thread B in folio_migrate_mapping(). In such a case,
>> folio_undo_large_rmappable() would remove folio A from the local list without
>> *any* lock protection, creating a race condition with the local list iteration
>> in deferred_split_scan().
>
> It's a good doubt to raise, but I think we're okay: because Kirill
> designed the deferred split list (and its temporary local list) to
> be safe in that way.
>
> You're right that if thread B's freeze to 0 wins the race, thread B
> will be messing with a list on thread A's stack while thread A is
> quite possibly running; but thread A will not leave that stack frame
> without taking again the split_queue_lock which thread B holds while
> removing from the list.
>
> We would certainly not want to introduce such a subtlety right now!
> But never mind page migration, this is how it has always worked when
> racing with the folio being freed at the same time - maybe
> deferred_split_scan() wins the freeing race and is the one to remove
> folio from deferred split list, or maybe the other freer does that.

Yes, thanks for explanation. And after thinking more, the
'list_for_each_entry_safe' in deferred_split_scan() can maintain the
list iteration safety, so I think this is safe.

> I forget whether there was an initial flurry of races to be fixed when
> it came in, but it has been stable against concurrent freeing for years.
>
> Please think this over again: do not trust my honeyed words!
>
>>
>> Anyway, I think this patch can still fix some possible races. Feel free to
>> add:
>> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>
> Thanks, but I certainly don't want this to go into the tree if it's
> still flawed as you suggest.

Now I have no doubt for this fix, and please continue to keep my
Reviewed-by tag, thanks.