On Thu, Jul 31, 2025 at 09:04:58AM -0400, Theodore Ts'o wrote: > On Tue, Jul 29, 2025 at 04:38:54PM -0700, Darrick J. Wong wrote: > > > > Just speaking for fuse2fs here -- that would be kinda nifty if libfuse > > could restart itself. It's unclear if doing so will actually enable us > > to clear the condition that caused the failure in the first place, but I > > suppose fuse2fs /does/ have e2fsck -fy at hand. So maybe restarts > > aren't totally crazy. > > I'm trying to understand what the failure scenario is here. Is this > if the userspace fuse server (i.e., fuse2fs) has crashed? If so, what > is supposed to happen with respect to open files, metadata and data > modifications which were in transit, etc.? Sure, fuse2fs could run > e2fsck -fy, but if there are dirty inode on the system, that's going > potentally to be out of sync, right? > > What are the recovery semantics that we hope to be able to provide? <echoing what we said on the ext4 call this morning> With iomap, most of the dirty state is in the kernel, so I think the new fuse2fs instance would poke the kernel with FUSE_NOTIFY_RESTARTED, which would initiate GETATTR requests on all the cached inodes to validate that they still exist; and then resend all the unacknowledged requests that were pending at the time. It might be the case that you have to that in the reverse order; I only know enough about the design of fuse to suspect that to be true. Anyhow once those are complete, I think we can resume operations with the surviving inodes. The ones that fail the GETATTR revalidation are fuse_make_bad'd, which effectively revokes them. All of this of course relies on fuse2fs maintaining as little volatile state of its own as possible. I think that means disabling the block cache in the unix io manager, and if we ever implemented delalloc then either we'd have to save the reservations somewhere or I guess you could immediately syncfs the whole filesystem to try to push all the dirty data to disk before we start allowing new free space allocations for new changes. --D > - Ted >
