Re:

["Yin, Fengwei" <fengwei.yin@intel.com>]

On 3/7/2024 10:05 PM, Matthew Wilcox wrote:
> On Thu, Mar 07, 2024 at 09:50:09PM +0800, Yin, Fengwei wrote:
>>
>>
>> On 3/7/2024 4:56 PM,  wrote:
>>> I just want to make sure I've understood correctly: CPU1's folio_put()
>>> is not the last reference, and it keeps iterating through the local
>>> list. Then CPU2 does the final folio_put() which causes list_del_init()
>>> to modify the local list concurrently with CPU1's iteration, so CPU1
>>> probably goes into the weeds?
>>
>> My understanding is this can not corrupt the folio->deferred_list as
>> this folio was iterated already.
> 
> I am not convinced about that at all.  It's possible this isn't the only
> problem, but deleting something from a list without holding (the correct)
> lock is something you have to think incredibly hard about to get right.
> I didn't bother going any deeper into the analysis once I spotted the
> locking problem, but the proof is very much on you that this is not a bug!
Removing folio from deferred_list in folio_put() also needs require
split_queue_lock. So my understanding is no deleting without hold
correct lock. local list iteration is impacted. But that's not the issue
Ryan hit here.

> 
>> But I did see other strange thing:
>> [   76.269942] page: refcount:0 mapcount:1 mapping:0000000000000000
>> index:0xffffbd0a0 pfn:0x2554a0
>> [   76.270483] note: kcompactd0[62] exited with preempt_count 1
>> [   76.271344] head: order:0 entire_mapcount:1 nr_pages_mapped:0 pincount:0
>>
>> This large folio has order 0? Maybe folio->_flags_1 was screwed?
>>
>> In free_unref_folios(), there is code like following:
>>                  if (order > 0 && folio_test_large_rmappable(folio))
>>                          folio_undo_large_rmappable(folio);
>>
>> But with destroy_large_folio():
>>          if (folio_test_large_rmappable(folio))
>>
>> 			folio_undo_large_rmappable(folio);
>>
>> Can it connect to the folio has zero refcount still in deferred list
>> with Matthew's patch?
>>
>>
>> Looks like folio order was cleared unexpected somewhere.
> 
> No, we intentionally clear it:
> 
> free_unref_folios -> free_unref_page_prepare -> free_pages_prepare ->
> page[1].flags &= ~PAGE_FLAGS_SECOND;
> 
> PAGE_FLAGS_SECOND includes the order, which is why we have to save it
> away in folio->private so that we know what it is in the second loop.
> So it's always been cleared by the time we call free_page_is_bad().
Oh. That's the key. Thanks a lot for detail explanation.

I thought there was a bug in other place, covered by
destroy_large_folio() but exposed by free_unref_folios()...


Regards
Yin, Fengwei