Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb_range

[James Houghton]

On Wed, Feb 1, 2023 at 7:56 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 01.02.23 16:45, James Houghton wrote:
> > On Tue, Jan 31, 2023 at 5:24 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >>
> >> On Tue, Jan 31, 2023 at 04:24:15PM -0800, James Houghton wrote:
> >>> On Mon, Jan 30, 2023 at 1:14 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >>>>
> >>>> On Mon, Jan 30, 2023 at 10:38:41AM -0800, James Houghton wrote:
> >>>>> On Mon, Jan 30, 2023 at 9:29 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >>>>>>
> >>>>>> On Fri, Jan 27, 2023 at 01:02:02PM -0800, James Houghton wrote:
> > [snip]
> >>>>>> Another way to not use thp mapcount, nor break smaps and similar calls to
> >>>>>> page_mapcount() on small page, is to only increase the hpage mapcount only
> >>>>>> when hstate pXd (in case of 1G it's PUD) entry being populated (no matter
> >>>>>> as leaf or a non-leaf), and the mapcount can be decreased when the pXd
> >>>>>> entry is removed (for leaf, it's the same as for now; for HGM, it's when
> >>>>>> freeing pgtable of the PUD entry).
> >>>>>
> >>>>> Right, and this is doable. Also it seems like this is pretty close to
> >>>>> the direction Matthew Wilcox wants to go with THPs.
> >>>>
> >>>> I may not be familiar with it, do you mean this one?
> >>>>
> >>>> https://lore.kernel.org/all/Y9Afwds%2FJl39UjEp@xxxxxxxxxxxxxxxxxxxx/
> >>>
> >>> Yep that's it.
> >>>
> >>>>
> >>>> For hugetlb I think it should be easier to maintain rather than any-sized
> >>>> folios, because there's the pgtable non-leaf entry to track rmap
> >>>> information and the folio size being static to hpage size.
> >>>>
> >>>> It'll be different to folios where it can be random sized pages chunk, so
> >>>> it needs to be managed by batching the ptes when install/zap.
> >>>
> >>> Agreed. It's probably easier for HugeTLB because they're always
> >>> "naturally aligned" and yeah they can't change sizes.
> >>>
> >>>>
> >>>>>
> >>>>> Something I noticed though, from the implementation of
> >>>>> folio_referenced()/folio_referenced_one(), is that folio_mapcount()
> >>>>> ought to report the total number of PTEs that are pointing on the page
> >>>>> (or the number of times page_vma_mapped_walk returns true). FWIW,
> >>>>> folio_referenced() is never called for hugetlb folios.
> >>>>
> >>>> FWIU folio_mapcount is the thing it needs for now to do the rmap walks -
> >>>> it'll walk every leaf page being mapped, big or small, so IIUC that number
> >>>> should match with what it expects to see later, more or less.
> >>>
> >>> I don't fully understand what you mean here.
> >>
> >> I meant the rmap_walk pairing with folio_referenced_one() will walk all the
> >> leaves for the folio, big or small.  I think that will match the number
> >> with what got returned from folio_mapcount().
> >
> > See below.
> >
> >>
> >>>
> >>>>
> >>>> But I agree the mapcount/referenced value itself is debatable to me, just
> >>>> like what you raised in the other thread on page migration.  Meanwhile, I
> >>>> am not certain whether the mapcount is accurate either because AFAICT the
> >>>> mapcount can be modified if e.g. new page mapping established as long as
> >>>> before taking the page lock later in folio_referenced().
> >>>>
> >>>> It's just that I don't see any severe issue either due to any of above, as
> >>>> long as that information is only used as a hint for next steps, e.g., to
> >>>> swap which page out.
> >>>
> >>> I also don't see a big problem with folio_referenced() (and you're
> >>> right that folio_mapcount() can be stale by the time it takes the
> >>> folio lock). It still seems like folio_mapcount() should return the
> >>> total number of PTEs that map the page though. Are you saying that
> >>> breaking this would be ok?
> >>
> >> I didn't quite follow - isn't that already doing so?
> >>
> >> folio_mapcount() is total_compound_mapcount() here, IIUC it is an
> >> accumulated value of all possible PTEs or PMDs being mapped as long as it's
> >> all or part of the folio being mapped.
> >
> > We've talked about 3 ways of handling mapcount:
> >
> > 1. The RFC v2 way, which is head-only, and we increment the compound
> > mapcount for each PT mapping we have. So a PTE-mapped 2M page,
> > compound_mapcount=512, subpage->_mapcount=0 (ignoring the -1 bias).
> > 2. The THP-like way. If we are fully mapping the hugetlb page with the
> > hstate-level PTE, we increment the compound mapcount, otherwise we
> > increment subpage->_mapcount.
> > 3. The RFC v1 way (the way you have suggested above), which is
> > head-only, and we increment the compound mapcount if the hstate-level
> > PTE is made present.
> >
> > With #1 and #2, there is no concern with folio_mapcount(). But with
> > #3, folio_mapcount() for a PTE-mapped 2M page mapped in a single VMA
> > would yield 1 instead of 512 (right?). That's what I mean.
>
> My 2 cents:
>
> The mapcount is primarily used (in hugetlb context) to
>
> (a) Detect if a page might be shared. mapcount > 1 implies that two
> independent page table hierarchies are mapping the page. We care about
> mapcount == 1 vs. mapcount != 1.
>
> (b) Detect if unmapping was sucessfull. We care about mapcount == 0 vs.
> mapcount != 0.
>
> For hugetlb, I don't see why we should care about the subpage mapcount
> at all.

Agreed -- it shouldn't really matter all that much.

>
> For (a) it's even good to count "somehow mapped into a single page table
> structure" as "mapcount == 1" For (b), we don't care as long as "still
> mapped" implies "mapcount != 0".

Thanks for your thoughts, David. So it sounds like you're still
squarely in the #3 camp. :)