On Sat, Jun 29, 2019 at 05:38:03AM +0100, Al Viro wrote: > PS: the problem is not gone in the next iteration of the patchset in > question. The patch I'm proposing (including dput_to_list() and _ONLY_ > compile-tested) follows. Comments? FWIW, there's another unpleasantness in the whole thing. Suppose we have picked a page full of dentries, all with refcount 0. We decide to evict all of them. As it turns out, they are from two filesystems. Filesystem 1 is NFS on a server, with currently downed hub on the way to it. Filesystem 2 is local. We attempt to evict an NFS dentry and get stuck - tons of dirty data with no way to flush them on server. In the meanwhile, admin tries to unmount the local filesystem. And gets stuck as well, since umount can't do anything to its dentries that happen to sit in our shrink list. I wonder if the root of problem here isn't in shrink_dcache_for_umount(); all it really needs is to have everything on that fs with refcount 0 dragged through __dentry_kill(). If something had been on a shrink list, __dentry_kill() will just leave behind a struct dentry completely devoid of any connection to superblock, other dentries, filesystem type, etc. - it's just a piece of memory that won't be freed until the owner of shrink list finally gets around to it. Which can happen at any point - all they'll do to it is dentry_free(), and that doesn't need any fs-related data structures. The logics in shrink_dcache_parent() is collect everything evictable into a shrink list if anything found - kick it out and repeat the scan otherwise, if something had been on other's shrink list repeat the scan I wonder if after the "no evictable candidates, but something on other's shrink lists" we ought to do something along the lines of rcu_read_lock walk it, doing if dentry has zero refcount if it's not on a shrink list, move it to ours else store its address in 'victim' end the walk if no victim found rcu_read_unlock else lock victim for __dentry_kill rcu_read_unlock if it's still alive if it's not IS_ROOT if parent is not on shrink list decrement parent's refcount put it on our list else decrement parent's refcount __dentry_kill(victim) else unlock if our list is non-empty shrink_dentry_list on it in there...
