Re: Splitting pinned folios

[Jane Chu <jane.chu@oracle.com>]

On 3/13/2024 9:53 AM, David Hildenbrand wrote:

>>   * 1 reference if the folio has private data (thank you, bufferheads)
>>
>> Oh.  Is that the bug?  can_split_folio() doesn't know that. This
>> usually isn't a problem because, eg, truncate_inode_partial_folio()
>> will remove the private data before calling split_folio().  But
>> memory-failure doesn't know about that rule ...
>
> The worst thing that could happen is that splitting the folio would 
> fail (because one more unexpected reference), not that we would split 
> where we shouldn't, right?
>
>>
>> No, that's not it.  shmem doesn't use the folio private flag. It's
>> still a bug, but it's not Jane's bug.
>
> Maybe *something* really accidentally dropped a page reference :/ 
> Reproducer + details would be great.
>
Thank you Matthew and David,  let me take some time digesting points you 
raised, and rerun the test, hopefully not only on my hacked up kernel 
for emulating GUP pin, but with real mr_register(), and report my findings.

thanks!

-jane