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?
