On 3/31/22 12:45, Al Viro wrote: > 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? Thanks for this catch. Since d_move holds the parent's lock, it should be possible to include the same condition as dentry_unlist() to ensure the cursor gets advanced if necessary. I could make it a small inline helper to make things easier to read. I will go ahead and fix that. Thanks, Stephen
