On Mon, Nov 25, 2024 at 12:05:29AM +0100, Mateusz Guzik wrote: > On Sun, Nov 24, 2024 at 11:10 PM Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > So I mention the "rename and extend i_size_seqcount" as a solution > > that I suspect might be acceptable if somebody has the motivation and > > energy, but honestly I also think "nobody can be bothered" is > > acceptable in practice. > > > > So happens recently the metadata ordeal also came up around getattr > where a submitter wanted to lock the inode around it. The posting Linus had been replying to: https://lore.kernel.org/all/20241124215014.GA3387508@ZenIV/ > Until the day comes when someone has way too much time on their hands > and patches it up (even that may encounter resistance though), I do > think it would make sense to nicely write it down somewhere so for > easy reference -- maybe as a comment above getattr and note around > other places like the timespec helpers to read that. See above. For those who'd missed the getattr thread - the approach proposed and NAKed there was to take ->i_rwsem (shared) in stat(2). A non-starter for obvious reasons, IMO. Seqcount avoids those, but it would need to be a pair of primitives used around the stores, with i_size_write() *not* usable inside such scope. Potential problems would be the amount of time spent inside those scopes and amount of spinning it would cause on the stat(2) side + the inode bloat. All of that is modulo usefulness of such atomicity - nothing mentioned so far seems to be a good reason to bother with all of that in the first place...
