[$] Dealing with negative dentries

The problem of negative dentries accumulating in the dentry cache in anunbounded manner, as we looked at back inApril, came up at the2022 Linux Storage,Filesystem, Memory-management and BPF Summit (LSFMM).Negative dentries reflect failed file-name lookups, which are then cached,saving an expensive operation if the file name in question is looked upagain. There is no mechanism to proactively prune back those cacheentries, however, so the cache keeps growing until memory pressure finallycauses the system to forcibly evict some of them, which can make the systemunresponsive for a long time or even cause a soft lockup.