On Tue, Jun 27, 2023 at 12:16:34PM +0100, Matthew Wilcox wrote: > This might be a good point to share that I'm considering (eventually) > not taking the folio lock here. > > My plan looks something like this (not fully baked): > > truncation (and similar) paths currently lock the folio, They would both > lock the folio _and_ claim that they were doing writeback on the folio. > > Filesystems would receive the folio from the writeback iterator with > the writeback flag already set. > > > This allows, eg, folio mapping/unmapping to take place completely > independent of writeback. That seems like a good thing; I can't see > why the two should be connected. Ah, i_size is a problem. With an extending write, i_size is updated while holding the folio lock. If we're writing back a partial folio, we zero the tail. That must not race with an extending write. So either we'd need to take both the folio lock & wb_lock when updating i_size, or we'd need to take both the lock and wb_lock when writing back the last page of a file.
