Re: Improve page refcount scalability

[Linus Torvalds]

On Wed, 14 Oct 2026 at 10:44, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
>
> I already have it:

So that was my thinking too, but I have my notes (from the original
truncage/hole punch patch) about mapping_exiting() being problematic
due to f2fs playing games with it.

Now, that's probably just a f2fs bug waiting to happen independently
of this. But the path is

ioctl(F2FS_IOC_START_ATOMIC_REPLACE) ->
  f2fs_ioc_start_atomic_write() ->
    truncate_inode_pages_final()

and that truncate_inode_pages_final() is the problem - it's what sets
mapping_set_exiting() on a live inode.

I had this as a note for the original branch, and it became a
non-issue with the change to the "set PG_rcu_free on hot pages in the
read path" instead.

But your change makes it an issue again.

Now, I think f2fs really is just buggy here. Calling
truncate_inode_pages_final() on something that is still live
afterwards is wrong - because it's not "final". As far as I can tell,
there may still be hundreds of active readers holding that fd open.

I might be wrong. Adding Jaegeuk to the cc.

                 Linus