On Fri, Feb 03, 2023 at 09:36:53AM +1100, Dave Chinner wrote: > On Wed, Feb 01, 2023 at 01:01:47PM -0300, Marcelo Tosatti wrote: > > > > umount calls invalidate_bh_lrus which IPIs each > > via invalidate_bdev(). So this is only triggered on unmount of > filesystems that use the block device mapping directly, right? > > Or is the problem that userspace is polling the block device (e.g. > udisks, blkid, etc) whilst the filesystem is mounted and populating > the block device mapping with cached pages so invalidate_bdev() > always does work even when the filesystem doesn't actually use the > bdev mapping? > > > CPU that has non empty per-CPU buffer_head cache: > > > > on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1); > > > > This interrupts CPUs which might be executing code sensitive > > to interferences. > > > > To avoid the IPI, free the per-CPU caches remotely via RCU. > > Two bh_lrus structures for each CPU are allocated: one is being > > used (assigned to per-CPU bh_lru pointer), and the other is > > being freed (or idle). > > Rather than adding more complexity to the legacy bufferhead code, > wouldn't it be better to switch the block device mapping to use > iomap+folios and get rid of the use of bufferheads altogether? Pretty sure ext4's journalling relies on the blockdev using buffer_heads. At least, I did a conversion of blockdev to use mpage_readahead() and ext4 stopped working.
