Public bug reported:

SRU Justification

Impact:
Currently shiftfs limits the maximum size for fallocate() needlessly causing 
calls such as fallocate --length 2GB ./file to fail. This limitation is 
arbitrary since it's not caused by the underlay but rather by shiftfs itself 
capping the s_maxbytes. This causes bugs such as the one reported in 
https://github.com/lxc/lxd/issues/6333.

Fix:
Currectly set up s_maxbytes when creating the shiftfs superblock.

Regression Potential:
Limited to shiftfs.

Test Case:
Try fallocate --length 3GB ./file on top of a filesystem with fallocate support 
on a fixed kernel and see that the call succeeds and the file is of the 
expected size.

Target Kernels:
All LTS kernels with shiftfs support.

** Affects: linux (Ubuntu)
     Importance: Undecided
     Assignee: Christian Brauner (cbrauner)
         Status: In Progress

** Changed in: linux (Ubuntu)
     Assignee: (unassigned) => Christian Brauner (cbrauner)

** Changed in: linux (Ubuntu)
       Status: New => In Progress

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https://bugs.launchpad.net/bugs/1849482

Title:
  shiftfs: fix fallocate()

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