On Fri, Apr 11, 2025 at 11:40:28AM +0200, Miklos Szeredi wrote:
> There are reports of soflockups in fsnotify if there are large numbers
> of negative dentries (e.g. ~300M) in a directory. This can happen if
> lots of temp files are created and removed and there's not enough
> memory pressure to trigger the lru shrinker.
>
> These are on old kernels and some of this is possibly due to missing
> 172e422ffea2 ("fsnotify: clear PARENT_WATCHED flags lazily"), but I
> managed to reproduce the softlockup on a recent kernel in
> fsnotify_set_children_dentry_flags() (see end of mail).
>
> This was with ~1.2G negative dentries. Doing "rmdir testdir"
> afterwards does not trigger the softlockup detector, due to the
> reschedules in shrink_dcache_parent() code, but it took 10 minutes(!)
> to finish removing that empty directory.
>
I wrote about this some time ago:
https://lore.kernel.org/linux-fsdevel/f7bp3ggliqbb7adyysonxgvo6zn76mo4unroagfcuu3bfghynu@7wkgqkfb5c43/#t
bottom line is only a small subset of negative entries is useful in the
long run
while a great policy to tame the total count while not hindering
performance is left as an exercise for the reader(tm), I outlined
something which should be *tolerable*.
