Re: [PATCH v4 35/36] mm: Convert do_set_pte() to set_pte_range()

[Yin Fengwei]

On 3/25/23 01:23, Will Deacon wrote:
> On Fri, Mar 24, 2023 at 03:11:00PM +0000, Matthew Wilcox wrote:
> > On Fri, Mar 24, 2023 at 02:58:29PM +0000, Will Deacon wrote:
> > > Yes, please don't fault everything in as young as it has caused horrible
> > > vmscan behaviour leading to app-startup slowdown in the past:
> > >
> > > https://lore.kernel.org/all/20210111140149.GB7642@willie-the-truck/
> > >
> > > If we have to use the same value for all the ptes, then just base them
> > > all on arch_wants_old_prefaulted_pte() as iirc hardware AF was pretty
> > > cheap in practice for us.
> >
> > I think that's wrong, because this is a different scenario.
> >
> > Before:
> >
> > We faulted in N single-page folios.  Each page/folio is tracked
> > independently.  That's N entries on whatever LRU list it ends up on.
> > The prefaulted ones _should_ be marked old -- they haven't been
> > accessed; we've just decided to put them in the page tables to
> > speed up faultaround.  The unaccessed pages need to fall off the LRU
> > list as quickly as possible; keeping them around only hurts if the
> > workload has no locality of reference.
> >
> > After:
> >
> > We fault in N folios, some possibly consisting of multiple pages.
> > Each folio is tracked separately, but individual pages in the folio
> > are not tracked; they belong to their folio.  In this scenario, if
> > the other PTEs for pages in the same folio are marked as young or old
> > doesn't matter; the entire folio will be tracked as young, because we
> > referenced one of the pages in this folio.  Marking the other PTEs as
> > young actually helps because we don't take pagefaults on them (whether
> > we have a HW or SW accessed bit).
> >
> > (can i just say that i dislike how we mix up our old/young accessed/not
> > terminology here?)
> >
> > We should still mark the PTEs referencing unaccessed folios as old.
> > No argument there, and this patch does that.  But it's fine for all the
> > PTEs referencing the accessed folio to have the young bit, at least as
> > far as I can tell.
>
> Ok, thanks for the explanation. So as long as
> arch_wants_old_prefaulted_pte() is taken into account for the unaccessed
> folios, then I think we should be good? Unconditionally marking those
> PTEs as old probably hurts x86.
Yes. We do only mark PTEs old for arch_wants_old_prefaulted_pte()
system. Thanks.


Regards
Yin, Fengwei

> Will