Re: [PATCH v19 00/40] DEPT(DEPendency Tracker)

[David Hildenbrand (Arm)]

>>> It's interesting, that for writeback we never unlock the folio on the I/O path,
>>> but instead have the dedicated page flag to tell us when writeback is complete.
>>
>> PG_locked and PG_writeback are two different lock-bits with two
>> different meanings.
>> When a folio is PG_locked the content is invalid and shouldn't be accessed.
>> When a folio is in PG_writeback its data is not safe even though
>> PG_dirty has been cleared (if I remember correctly).
>>
>> So these are two separate, though related, lock bits that would both
>> need to be tracked.  lockdep would detect how they are related.
> 
> Some history ... before we had PG_writeback, we held PG_locked
> throughout the writeback process.  People argued convincingly that it
> was fine to map a page into a process's address space (which requires
> PG_locked) while it was being written back.  And so PG_locked was
> split.  Now we take PG_locked at the beginning of the writeback process
> and downgrade to PG_writeback at some point.

Right, conceptually I would have thought we could do something similar on the
readahead path. PG_uptodate is likely not what we want I suspect.

Not that such a change would be easy, or that we should do it; just one thing
that came to mind.

-- 
Cheers,

David