On Thu, Feb 13, 2020 at 01:30:11PM -0800, Linus Torvalds wrote: > On Wed, Feb 12, 2020 at 9:55 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > > What I don't understand is the insistence on getting those dentries > > via dcache lookups. > > I don't think that's an "insistence", it's more of a "historical > behavior" together with "several changes over the years to deal with > dentry-level cleanups and updates". > > > _IF_ we are willing to live with cacheline > > contention (on ->d_lock of root dentry, if nothing else), why not > > do the following: > > * put all dentries of such directories ([0-9]* and [0-9]*/task/*) > > into a list anchored in task_struct; have non-counting reference to > > task_struct stored in them (might simplify part of get_proc_task() users, > > Hmm. > > Right now I don't think we actually create any dentries at all for the > short-lived process case. > > Wouldn't your suggestion make fork/exit rather worse? > > Or would you create the dentries dynamically still at lookup time, and > then attach them to the process at that point? > > What list would you use for the dentry chaining? Would you play games > with the dentry hashing, and "hash" them off the process, and never > hit in the lookup cache? I'd been thinking of ->d_fsdata pointing to a structure with list_head and a (non-counting) task_struct pointer for those guys. Allocated on lookup, of course (as well as readdir ;-/) and put on the list at the same time. IOW, for short-lived process we simply have an empty (h)list anchored in task_struct and that's it.
