On Thu, 29 Jun 2023 12:22:24 -0300 Jason Gunthorpe <jgg@xxxxxxxx> wrote: > On Wed, Jun 28, 2023 at 10:08:08PM -0700, Hugh Dickins wrote: > > On Wed, 28 Jun 2023, Gerald Schaefer wrote: > > > > > > As discussed in the other thread, we would rather go with less complexity, > > > possibly switching to an approach w/o the list and fragment re-use in the > > > future. For now, as a first step in that direction, we can try with not > > > adding fragments back only for pte_free_defer(). Here is an adjusted > > > version of your patch, copying most of your pte_free_defer() logic and > > > also description, tested with LTP and all three of your patch series applied: > > > > Thanks, Gerald: I don't mind abandoning my 13/12 SLAB_TYPESAFE_BY_RCU > > patch (posted with fewer Cc's to the s390 list last week), and switching > > to your simpler who-cares-if-we-sometimes-don't-make-maximal-use-of-page > > patch. > > > > But I didn't get deep enough into it today to confirm it - and disappointed > > that you've found it necessary to play with pt_frag_refcount in addition to > > _refcount and HH bits. No real problem with that, but my instinct says it > > should be simpler. Yes, I also found it a bit awkward, but it seemed "good and simple enough", to have something to go forward with, while my instinct was in line with yours. > > Is there any reason it should be any different at all from what PPC is > doing? > > I still think the right thing to do here is make the PPC code common > (with Hugh's proposed RCU modification) and just use it in both > arches.... With the current approach, we would not add back fragments _only_ for the new pte_free_defer() path, while keeping our cleverness for the other paths. Not having a good overview of the negative impact wrt potential memory waste, I would rather take small steps, if possible. If we later switch to never adding back fragments, of course we should try to be in line with PPC implementation.
