On Thu, Oct 08, 2020 at 07:54:09PM -0700, Darrick J. Wong wrote: > On Thu, Oct 08, 2020 at 03:38:58PM -0700, Josh Triplett wrote: > > On Thu, Oct 08, 2020 at 10:54:48AM -0700, Darrick J. Wong wrote: > > > > > the head", and continued with the notion that anything other than > > > > > e2fsprogs making something to be mounted by mount(2) and handled by > > > > > fs/ext4 is being "inflicted", and if the goal didn't still seem to be > > > > > "how do we make it go away so that only e2fsprogs and the kernel ever > > > > > touch ext4". I started this thread because I'd written some userspace > > > > > code, a new version of the kernel made that userspace code stop working, > > > > > so I wanted to report that the moment I'd discovered that, along with a > > > > > potential way to address it with as little disrupton to ext4 as > > > > > possible. > > > > > > Ted: I don't think it's a good idea to make SHARED_BLOCKS disable block > > > validity checking by default. You could someday enable users to write > > > to block-sharing files by teaching ext4 how to COW, at which point you'd > > > need correct bitmaps and block validity to prevent accidental overwrite > > > of critical metadata. At that point you'd either be stuck with the > > > precedent that SHARED_BLOCKS implies noblock_validity, or you'd end up > > > breaking Josh's images again. > > > > That's a fair point (though I think a writable COW version of > > SHARED_BLOCKS would need a different flag). It'd make more sense to key > > this logic off of RO_COMPAT_READONLY or similar. But even better: > > It wouldn't require a new flag -- "rocompat" features bits mean that > "it's safe to allow userspace to read files off the disk if software > doesn't recognize this feature bit". Sure; I was more thinking that if that involved adding some data structures to track shared blocks and the need to COW, whatever mechanism that used might potentially need an incompat flag. > If someone /did/ add the code to write to files safely in the presence > of shared blocks, all they'd have to do to light up the functionality is > add it to the _SUPP define. Also, it's strange that the kernel source > doesn't even know of SHARED_BLOCKS, but that's also on Ted... It would be nice if the kernel's ext4.h header and e2fsprogs were fully in sync, yes. - Josh Triplett
