A slab allocator (removal) update

Posted May 23, 2023 5:34 UTC (Tue) by mokki (subscriber, #33200)
In reply to: A slab allocator (removal) update by brenns10
Parent article: A slab allocator (removal) update

I've maintained CI machines that have 50% of 384 GiB memory in negative dentries.
Often, when the kernel tried to free some more memory, the result was soft lockups of over 1 minute in dmesg.
Flushing manually the dentry cache regularly helped. This was ~6 years ago and hopefully there are now better limits to the max amount of negative dentries.

A slab allocator (removal) update

Posted May 23, 2023 18:03 UTC (Tue) by WolfWings (subscriber, #56790) [Link]

Still no maximum limits for negative dentries at all in any LTS kernels I'm aware of, and they get intermixed entirely with 'positive' dentries.

There's been numerous attempts to improve the situation over the years, LWN articles about it even, but it never makes progress since it's a relatively niche situation without enough visibility to most kernel devs to maintain momentum to improve the situation. I'm not even sure there's any mechanism to stop the dentry cache purge when memory-pressure triggers it... it may literally scan ALL of cache and purge what it can instead of freeing say a few MB and stopping, I haven't checked the code in depth but that's the general behavior I've seen in the past.