On Fri, Feb 24, 2023 at 12:16:49PM -0800, Linus Torvalds wrote: > On Fri, Feb 24, 2023 at 12:14 PM David Howells <dhowells@xxxxxxxxxx> wrote: > > > > Then why do we have to wait for PG_writeback to complete? > > At least for PG_writeback, it's about "the _previous_ dirty write is > still under way, but - since PG_dirty is set again - the page has been > dirtied since". > > So we have to start _another_ writeback, because while the current > writeback *might* have written the updated data, that is not at all > certain or clear. also, we only have a writeback bit, not a writeback count. And when the current writeback completes, it'll clear that bit. We're also being kind to our backing store and not writing to the same block twice at the same time. > I'm not sure what the fscache rules are. My understanding is that the fscache bit is set under several circumstances, but if the folio is dirty _and_ the fscache bit is set, it means the folio is currently being written to the cache device. I don't see a conflict there; we can write to the backing store and the cache device at the same time.
