Re: [PATCH] vfs: move dentry shrinking outside the inode lock in 'rmdir()'

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, May 13, 2024 at 08:58:33AM -0700, Linus Torvalds wrote:
 
> We *could* strive for a hybrid approach, where we handle the common
> case ("not a ton of child dentries") differently, and just get rid of
> them synchronously, and handle the "millions of children" case by
> unhashing the directory and dealing with shrinking the children async.

try_to_shrink_children()?  Doable, and not even that hard to do, but
as for shrinking async...  We can easily move it out of inode_lock
on parent, but doing that really async would either need to be
tied into e.g. remount r/o logics or we'd get userland regressions.

I mean, "I have an opened unlinked file, can't remount r/o" is one
thing, but "I've done rm -rf ~luser, can't remount r/o for a while"
when all luser's processes had been killed and nothing is holding
any of that opened... ouch.