Re: Mapcount of subpages

[Hugh Dickins]

On Thu, 23 Sep 2021, Zi Yan wrote:
> On 23 Sep 2021, at 19:48, Hugh Dickins wrote:
> > On Thu, 23 Sep 2021, Zi Yan wrote:
> >> On 23 Sep 2021, at 17:54, Yang Shi wrote:
> >>> On Thu, Sep 23, 2021 at 2:10 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> >>>>
> >>>> NR_FILE_MAPPED being used for /proc/meminfo's "Mapped:" and a couple
> >>>> of other such stats files, and for a reclaim heuristic in mm/vmscan.c.
> >>>>
> >>>> Allow ourselves more slack in NR_FILE_MAPPED accounting (either count
> >>>> each pte as if it mapped the whole THP, or don't count a THP's ptes
> >>>> at all - you opted for the latter in the "Mlocked:" accounting),
> >>>> and I suspect subpage _mapcount could be abandoned.
> >>>
> >>> AFAIK, partial THP unmap may need the _mapcount information of every
> >>> subpage otherwise the deferred split can't know what subpages could be
> >>> freed.
> >
> > 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.
> >
> >>
> >> Could we just scan page tables of a THP during deferred split process
> >> instead? Deferred split is a slow path already, so maybe it can afford
> >> the extra work.
> >
> > But unless I misunderstand, actually carrying out the deferred split
> > already unmaps, uses migration entries, and remaps the remaining ptes:
> > needing no help from subpage _mapcounts to do those, and free the rest.
> 
> You are right. unmap_page() during THP split is scanning the page tables
> already.
> 
> For deciding whether to queue a THP for deferred split, we probably can
> keep PageDoubleMap bit to indicate if any subpage is PTE mapped.

Maybe, maybe not.

> 
> But without subpage _mapcount, detecting extra pins to a THP before split
> might be not as easy as with it. This means every THP split will need to
> perform unmap_page(), then check the remaining page_count to see if
> THP split is possible. That would also introduce extra system-wide overheads
> from unmapping pages. Am I missing anything?

I did not explain clearly enough: a subpage's ptes must still be counted
in total_mapcount(); but I'm suggesting that perhaps they can be counted
all together (either in the head page's _mapcount, or in a separate field
if that works better), instead of being distributed amongst the separate
subpages' _mapcounts.

And this would lower the system-wide overheads inside total_mapcount()
and page_mapped() (and maybe others).

Hugh