Re: bad things when too many negative dentries in a directory

[Matthew Wilcox <willy@infradead.org>]

On Wed, Apr 16, 2025 at 05:18:17PM +0200, Miklos Szeredi wrote:
> Lack of memory pressure should mean that nobody else needs that
> memory, so it should make no difference if it's used up in negative
> dentries instead of being free memory.  Maybe I'm missing something
> fundamental?

You're missing two things:

 - The dentry hash table is a fixed size.  Long chains give poor
   performance, so polluting the hash table with unused entries
   has a cost.
 - Eventually, we do trigger reclaim.  And then we wait for hours while
   the reclaiming process tries to shrink billions of entries.  I think
   we had a report on one machine of it taking more than 24 hours ("more
   than" because the customer decided enough was enough and rebooted the
   machine).