On Fri, Apr 16, 2021 at 04:00:48PM +0800, Zhang Yi wrote: > Now, we use already use "if (bdev->bd_super)" to prevent call into > ->bdev_try_to_free_page unless the super is alive, and the problem is > bd_super becomes NULL concurrently after this check. So, IIUC, I think it's > the same to switch to check the superblock is active or not. The acvive > flag also could becomes inactive (raced by umount) after we call into > bdev_try_to_free_page(). Indeed. > In order to close this race, One solution is introduce a lock to synchronize > the active state between kill_block_super() and blkdev_releasepage(), but > the releasing page process have to try to acquire this lock in > blkdev_releasepage() for each page, and the umount process still need to wait > until the page release if some one invoke into ->bdev_try_to_free_page(). > I think this solution may affect performace and is not a good way. > Think about it in depth, use percpu refcount seems have the smallest > performance effect on blkdev_releasepage(). > > If you don't like the refcount, maybe we could add synchronize_rcu_expedited() > in ext4_put_super(), it also could prevent this race. Any suggestions? I really don't like to put a lot of overhead into the core VFS and block device code. ext4/jbd does not own the block device inode and really has no business controlling releasepage for it. I suspect the right answer might be to simply revert the commit that added this hook.
