On Tue, Feb 18, 2025 at 12:23:27PM -0800, Darrick J. Wong wrote: > On Mon, Feb 17, 2025 at 10:31:28AM +0100, Christoph Hellwig wrote: > > The buffer cache keeps a bt_io_count per-CPU counter to track all > > in-flight I/O, which is used to ensure no I/O is in flight when > > unmounting the file system. > > > > For most I/O we already keep track of inflight I/O at higher levels: > > > > - for synchronous I/O (xfs_buf_read/xfs_bwrite/xfs_buf_delwri_submit), > > the caller has a reference and waits for I/O completions using > > xfs_buf_iowait > > - for xfs_buf_delwri_submit the only caller (AIL writeback) tracks the > > Do you mean xfs_buf_delwri_submit_nowait here? Yes. > IOWs, only asynchronous readahead needs an explicit counter in the > xfs_buf to prevent unmount because: > > 0. Anything done in mount/unmount/freeze holds s_umount > 1. Buffer reads done on behalf of a file hold the file open and pin the > mount > 2. Dirty buffers have log items, and we can't unmount until those are > dealt with > 3. Fsck holds an open fd and hence pins the mount > 4. Unmount blocks until background gc finishes > > Right? Yes. > I almost wonder if you could just have a percpu counter in the > xfs_mount but that sounds pretty hot. Well, that would remove the nice xfs_buftarg_wait() abstraction. Givne that we don't even allocate an extra buftrag unless we use it that doesn't seem very useful. > > + /* there are currently no valid flags for xfs_buf_get_uncached */ > > + ASSERT(flags == 0); > > Can we just get rid of flags then? AFAICT nobody uses it either here or > in xfsprogs, and in fact I think there's a nasty bug in the userspace > rtsb code: See my reply to the last patch: I actually have a patch to remove it, but it conflicts with the zoned series. So for now I'll defer it until that is merged.
