VFS cache: Make changes to 'ncsize' update 'neg_min'

[olce]

Summary

'neg_min' (sysctl 'vfs.cache.param.negmin', R/O) is the minimum number of
negative entries in the cache before it will consider evicting some negative
ones. It is sized with respect to 'ncsize' (total allowed cache capacity,
sysctl 'vfs.cache.param.size') by applying to it the percentage in
'ncnegminpct':(sysctl 'vfs.cache.param.negminpct').

Changing 'ncnegminpct' causes 'neg_min' to be recomputed, but changing 'ncsize'
would not. Ensure it is in the latter case as well.

Sponsored by: The FreeBSD Foundation

[mjg]

handling of this sysctl is already broken, it needs to be combined with cache_changesize

[mjg]

Looking at this more it was clearly a bug to have it RW to begin with, fixed here: https://cgit.FreeBSD.org/src/commit/?id=33fdf1afaea931eaa5dd14e6c097d7ffafe2fb1a

I don't think a dedicated sysctl to just resize the namecache warrants any work to be supported.

[olce]

I had seen indeed that changing ncsize doesn't resize the hash. I thought that this could be deliberate on your side (e.g., to test the behavior with different hash table load factors).

But my goal with this change was to ease a bit the scenario where I want to disable the cache for some experiments, by making sure no entry will be added again during the test.

So I'll come up with other changes, most probably new knobs. I can put them under debug if you prefer.

[mjg]

Have you seen debug.vfscache (hidden behind DEBUG_CACHE) and the vfs.cache_fast_lookup sysctls?