Re: [RFC 2/2] mm: Defer TLB flush by keeping both src and dst folios at migration

[Byungchul Park <byungchul@sk.com>]

On Tue, Aug 15, 2023 at 09:27:26AM +0800, Huang, Ying wrote:
> Byungchul Park <byungchul@sk.com> writes:
> 
> > Implementation of CONFIG_MIGRC that stands for 'Migration Read Copy'.
> >
> > We always face the migration overhead at either promotion or demotion,
> > while working with tiered memory e.g. CXL memory and found out TLB
> > shootdown is a quite big one that is needed to get rid of if possible.
> >
> > Fortunately, TLB flush can be defered or even skipped if both source and
> > destination of folios during migration are kept until all TLB flushes
> > required will have been done, of course, only if the target PTE entries
> > have read only permission, more precisely speaking, don't have write
> > permission. Otherwise, no doubt the folio might get messed up.
> >
> > To achieve that:
> >
> >    1. For the folios that have only non-writable TLB entries, prevent
> >       TLB flush by keeping both source and destination of folios during
> >       migration, which will be handled later at a better time.
> >
> >    2. When any non-writable TLB entry changes to writable e.g. through
> >       fault handler, give up CONFIG_MIGRC mechanism so as to perform
> >       TLB flush required right away.
> >
> >    3. TLB flushes can be skipped if all TLB flushes required to free the
> >       duplicated folios have been done by any reason, which doesn't have
> >       to be done from migrations.
> >
> >    4. Adjust watermark check routine, __zone_watermark_ok(), with the
> >       number of duplicated folios because those folios can be freed
> >       and obtained right away through appropreate TLB flushes.
> >
> >    5. Perform TLB flushes and free the duplicated folios pending the
> >       flushes if page allocation routine is in trouble due to memory
> >       pressure, even more aggresively for high order allocation.
> 
> Is the optimization restricted for page migration only?  Can it be used
> for other places?  Like page reclaiming?

I don't get you but if I'm missing something here and your idea is
something good, then it can be a future work.

	Byungchul