On 14:26 10/03, Matthew Wilcox wrote: > On Wed, Mar 10, 2021 at 08:21:59AM -0600, Goldwyn Rodrigues wrote: > > On 13:02 10/03, Matthew Wilcox wrote: > > > On Wed, Mar 10, 2021 at 07:30:41AM -0500, Neal Gompa wrote: > > > > Forgive my ignorance, but is there a reason why this isn't wired up to > > > > Btrfs at the same time? It seems weird to me that adding a feature > > > > > > btrfs doesn't support DAX. only ext2, ext4, XFS and FUSE have DAX support. > > > > > > If you think about it, btrfs and DAX are diametrically opposite things. > > > DAX is about giving raw access to the hardware. btrfs is about offering > > > extra value (RAID, checksums, ...), none of which can be done if the > > > filesystem isn't in the read/write path. > > > > > > That's why there's no DAX support in btrfs. If you want DAX, you have > > > to give up all the features you like in btrfs. So you may as well use > > > a different filesystem. > > > > DAX on btrfs has been attempted[1]. Of course, we could not > > But why? A completeness fetish? I don't understand why you decided > to do this work. If only I had a penny every time I heard "why would you want to do that?" > > > have checksums or multi-device with it. However, got stuck on > > associating a shared extent on the same page mapping: basically the > > TODO above dax_associate_entry(). > > > > Shiyang has proposed a way to disassociate existing mapping, but I > > don't think that is the best solution. DAX for CoW will not work until > > we have a way of mapping a page to multiple inodes (page->mapping), > > which will convert a 1-N inode-page mapping to M-N inode-page mapping. > > If you're still thinking in terms of pages, you're doing DAX wrong. > DAX should work without a struct page. Not pages specifically, but mappings. fsdax needs the mappings during the page fault and it breaks in case both files fault on the same shared extent. For Reference: WARN_ON_ONCE(page->mapping && page->mapping != mapping) in dax_disassociate_entry(). -- Goldwyn
