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

[David Hildenbrand]

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.

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,

David / dhildenb