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
> > > 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
> > 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. 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)