[LSF/MM TOPIC] Better handling of negative dentries

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The number of negative dentries is effectively constrained only by memory
size.  Systems which do not experience significant memory pressure for
an extended period can build up millions of negative dentries which
clog the dcache.  That can have different symptoms, such as inotify
taking a long time [1], high memory usage [2] and even just poor lookup
performance [3].  We've also seen problems with cgroups being pinned
by negative dentries, though I think we now reparent those dentries to
their parent cgroup instead.

We don't have a really good solution yet, and maybe some focused
brainstorming on the problem would lead to something that actually works.

(Apologies to Stephen; I should have thought to send this before the
invitations to LSFMM went out).

[1] https://lore.kernel.org/linux-fsdevel/20220209231406.187668-1-stephen.s.brennan@xxxxxxxxxx/
[2] https://lore.kernel.org/linux-fsdevel/1611235185-1685-1-git-send-email-gautham.ananthakrishna@xxxxxxxxxx/
[3] https://lore.kernel.org/linux-fsdevel/158893941613.200862.4094521350329937435.stgit@buzz/




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