On Fri, Oct 18, 2019 at 07:30:57AM -0700, Sean Christopherson wrote: > On Fri, Oct 18, 2019 at 03:49:42PM +0300, Jarkko Sakkinen wrote: > > On Wed, Oct 16, 2019 at 11:37:45AM -0700, Sean Christopherson wrote: > > > Track the free page count on a per EPC section basis so that the value > > > is properly protected by the section's spinlock. > > > > > > As was pointed out when the change was proposed[*], using a global > > > non-atomic counter to track the number of free EPC pages is not safe. > > > The order of non-atomic reads and writes are not guaranteed, i.e. > > > concurrent RMW operats can write stale data. This causes a variety > > > of bad behavior, e.g. livelocks because the free page count wraps and > > > causes the swap thread to stop reclaiming. > > > > > > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> > > > > What is the reason not change it just to atomic? > > The purpose of separate sections is to avoid bouncing locks and whatnot > across packages. Adding a global atomic to the hotpath defeats that > purpose. I do get that but it does not actually cause incorrect behaviour, right? Not being atomic obivously does because READ part of the READ+STORE can get re-ordered. > Can't the sysfs read helper aggregate the info? Sure. /Jarkko
