Re: Mapcount of subpages

[Yang Shi]

On Thu, Sep 23, 2021 at 6:32 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Sep 23, 2021 at 06:11:19PM -0700, Yang Shi wrote:
> > On Thu, Sep 23, 2021 at 4:49 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > > I believe Yang Shi is right insofar as the decision on whether it's worth
> > > queuing for deferred split is being done based on those subpage _mapcounts.
> > > That is a use I had not considered, and I've given no thought to how
> > > important or not it is.
> >
> > Anyway deferred split is anon THP specific. We don't have to worry
> > about this for file THP. So your suggestion about just counting total
> > mapcount seems feasible to me for file THP at least.
>
> But I think we probably *should* do deferred split for file THP.
> At the moment, when we truncate to the middle of a shmem THP, we try
> a few times to split it and then just give up.  We should probably try
> once and then queue it for deferred split.

Yes, probably. Anyway this doesn't need _mapcount of subpages.