Re: [RFC PATCH 2/2] fs/dcache: Add negative-dentry-ratio config

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Mar 31, 2022 at 12:08:27PM -0700, Stephen Brennan wrote:
> Negative dentry bloat is a well-known problem. For systems without
> memory pressure, some workloads (like repeated stat calls) can create an
> unbounded amount of negative dentries quite quickly. In the best case,
> these dentries could speed up a subsequent name lookup, but in the worst
> case, they are never used and their memory never freed.
> 
> While systems without memory pressure may not need that memory for other
> purposes, negative dentry bloat can have other side-effects, such as
> soft lockups when traversing the d_subdirs list or general slowness with
> managing them. It is a good idea to have some sort of mechanism for
> controlling negative dentries, even outside memory pressure.
> 
> This patch attempts to do so in a fair way. Workloads which create many
> negative dentries must create many dentries, or convert dentries from
> positive to negative. Thus, negative dentry management is best done
> during these same operations, as it will amortize its cost, and
> distribute the cost to the perpetrators of the dentry bloat. We
> introduce a sysctl "negative-dentry-ratio" which sets a maximum number
> of negative dentries per positive dentry, N:1. When a dentry is created
> or unlinked, the next N+1 dentries of the parent are scanned. If no
> positive dentries are found, then a candidate negative dentry is killed.

Er...  So what's to stop d_move() from leaving you with your cursor
pointer poiting into the list of children of another parent?

What's more, your dentry_unlist() logics will be defeated by that -
if victim used to have a different parent, got moved, then evicted,
it looks like you could end up with old parent cursor pointing
to the victim and left unmodified by dentry_unlist() (since it looks
only at the current parent's cursor).  Wait for it to be freed and
voila - access to old parent's cursor will do unpleasant things.

What am I missing here?