On Tue, Feb 14, 2023 at 02:56:04PM +0100, Jan Kara wrote: > On Mon 13-02-23 01:59:28, Christoph Hellwig wrote: > > Eww. The block bounc code really needs to go away, so a new user > > makes me very unhappy. > > > > But independent of that I don't think this is enough anyway. Just > > copying the data out into a new page in the block layer doesn't solve > > the problem that this page needs to be tracked as dirtied for fs > > accounting. e.g. every time we write this copy it needs space allocated > > for COW file systems. > > Right, I forgot about this in my RFC. My original plan was to not clear the > dirty bit in clear_page_dirty_for_io() even for WB_SYNC_ALL writeback when > we do writeback the page and perhaps indicate this in the return value of > clear_page_dirty_for_io() so that the COW filesystem can keep tracking this > page as dirty. I don't think this works, especially if the COW mechanism relies on delayed allocation to prevent ENOSPC during writeback. That is, we need a write() or page fault (to run ->page_mkwrite()) after every call to folio_clear_dirty_for_io() in the writeback path to ensure that new space is reserved for the allocation that will occur during a future writeback of that page. Hence we can't just leave the page dirty on COW filesystems - it has to go through a clean state so that the clean->dirty event can be gated on gaining the space reservation that allows it to be written back again. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
