On Thu, Mar 20, 2025 at 11:36:17AM -0400, James Bottomley wrote: > [Note: this got scheduled because I suggested it as a topic on the > application form. I'm still not convinced it's that relevant and it > could become a lightning talk instead if there's not much interest] > > This came up as a problem because efivarfs needs a hibernate hook. The > specific efivarfs problem is that when the kernel is running, it owns > the EFI runtime interface, so nothing can update variables without > going through the kernel and so the cache of efivarfs (held in the > variable dentries) reflects exactly the state of the underlying > variables. However, when the system is hibernated, other OSs could > boot and alter EFI variables, so the exact cache correspondence > guarantee is broken and efivarfs needs to resync the variable store on > resume from hibernate. > > Part of the thought here is that other filesystems might possibly want > suspend resume hooks as well, although not for the reasons efivarfs > does: Hibernate is a particularly risky operation and resume may not > work leading to a full reboot and filesystem inconsistencies. In many > ways, a failed resume is exactly like a system crash, for which > filesystems already make specific guarantees. However, it is a crash > for which they could, if they had power management hooks, be forewarned > and possibly make the filesystem cleaner for eventual full restore. > Things like guaranteeing that uncommitted data would be preserved even > if a resume failed, which isn't something we guarantee across a crash > today. > > The other reason for discussing this is that adding power management > hooks directly to efivarfs resulted in a lot of deadlock potential > (discussion of which is already on linux-fsdevel), which could > potentially be lessened by making it part of the VFS API. If other > filesystems think they might use it in future, we should discuss how > this could work. there's a hook in s_ops, although today it's only called from blk_holder ops - presumably you'd just want to add a superblock flag to say "I need freeze/suspend hooks to be called directly"?
