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

[James Houghton <jthoughton@google.com>]

On Wed, Feb 8, 2023 at 8:16 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Feb 07, 2023 at 04:26:02PM -0800, James Houghton wrote:
> > On Tue, Feb 7, 2023 at 3:13 PM Peter Xu <peterx@redhat.com> wrote:
> > >
> > > James,
> > >
> > > On Tue, Feb 07, 2023 at 02:46:04PM -0800, James Houghton wrote:
> > > > > Here is the result: [1] (sorry it took a little while heh). The
> > >
> > > Thanks.  From what I can tell, that number shows that it'll be great we
> > > start with your rfcv1 mapcount approach, which mimics what's proposed by
> > > Matthew for generic folio.
> >
> > Do you think the RFC v1 way is better than doing the THP-like way
> > *with the additional MMU notifier*?
>
> What's the additional MMU notifier you're referring?

An MMU notifier that informs KVM that a collapse has happened without
having to invalidate_range_start() and invalidate_range_end(), the one
you're replying to lower down in the email. :) [ see below... ]

>
> >
> > >
> > > > > implementation of the "RFC v1" way is pretty horrible[2] (and this
> > >
> > > Any more information on why it's horrible? :)
> >
> > I figured the code would speak for itself, heh. It's quite complicated.
> >
> > I really didn't like:
> > 1. The 'inc' business in copy_hugetlb_page_range.
> > 2. How/where I call put_page()/folio_put() to keep the refcount and
> > mapcount synced up.
> > 3. Having to check the page cache in UFFDIO_CONTINUE.
>
> I think the complexity is one thing which I'm fine with so far.  However
> when I think again about the things behind that complexity, I noticed there
> may be at least one flaw that may not be trivial to work around.
>
> It's about truncation.  The problem is now we use the pgtable entry to
> represent the mapcount, but the pgtable entry cannot be zapped easily,
> unless vma unmapped or collapsed.
>
> It means e.g. truncate_inode_folio() may stop working for hugetlb (of
> course, with page lock held).  The mappings will be removed for real, but
> not the mapcount for HGM anymore, because unmap_mapping_folio() only zaps
> the pgtable leaves, not the ones that we used to account for mapcounts.
>
> So the kernel may see weird things, like mapcount>0 after
> truncate_inode_folio() being finished completely.
>
> For HGM to do the right thing, we may want to also remove the non-leaf
> entries when truncating or doing similar things like a rmap walk to drop
> any mappings for a page/folio.  Though that's not doable for now because
> the locks that truncate_inode_folio() is weaker than what we need to free
> the pgtable non-leaf entries - we'll need mmap write lock for that, the
> same as when we unmap or collapse.
>
> Matthew's design doesn't have such issue if the ptes need to be populated,
> because mapcount is still with the leaves; not the case for us here.
>
> If that's the case, _maybe_ we still need to start with the stupid but
> working approach of subpage mapcounts.

Good point. I can't immediately think of a solution. I would prefer to
go with the subpage mapcount approach to simplify HGM for now;
optimizing mapcount for HugeTLB can then be handled separately. If
you're ok with this, I'll go ahead and send v2.

One way that might be possible: using the PAGE_SPECIAL bit on the
hstate-level PTE to indicate if mapcount has been incremented or not
(if the PTE is pointing to page tables). As far as I can tell,
PAGE_SPECIAL doesn't carry any meaning for HugeTLB PTEs, but we would
need to be careful with existing PTE examination code as to not
misinterpret these PTEs.

>
> [...]
>
> > > > > Matthew is trying to solve the same problem with THPs right now: [3].
> > > > > I haven't figured out how we can apply Matthews's approach to HGM
> > > > > right now, but there probably is a way. (If we left the mapcount
> > > > > increment bits in the same place, we couldn't just check the
> > > > > hstate-level PTE; it would have already been made present.)
> > >
> > > I'm just worried that (1) this may add yet another dependency to your work
> > > which is still during discussion phase, and (2) whether the folio approach
> > > is easily applicable here, e.g., we may not want to populate all the ptes
> > > for hugetlb HGMs by default.
> >
> > That's true. I definitely don't want to wait for this either. It seems
> > like Matthew's approach won't work very well for us -- when doing a
> > lot of high-granularity UFFDIO_CONTINUEs on a 1G page, checking all
> > the PTEs to see if any of them are mapped would get really slow.
>
> I think it'll be a common problem to userfaultfd when it comes, e.g.,
> userfaultfd by design is PAGE_SIZE based so far.  It needs page size
> granule on pgtable manipulations, unless we extend the userfaultfd protocol
> to support folios, iiuc.
>
> >
> > >
> > > > >
> > > > > We could:
> > > > > - use the THP-like way and tolerate ~1 second collapses
> > > >
> > > > Another thought here. We don't necessarily *need* to collapse the page
> > > > table mappings in between mmu_notifier_invalidate_range_start() and
> > > > mmu_notifier_invalidate_range_end(), as the pfns aren't changing,
> > > > we aren't punching any holes, and we aren't changing permission bits.
> > > > If we had an MMU notifier that simply informed KVM that we collapsed
> > > > the page tables *after* we finished collapsing, then it would be ok
> > > > for hugetlb_collapse() to be slow.

[ from above... ] This MMU notifier. :)

> > >
> > > That's a great point!  It'll definitely apply to either approach.
> > >
> > > >
> > > > If this MMU notifier is something that makes sense, it probably
> > > > applies to MADV_COLLAPSE for THPs as well.
> > >
> > > THPs are definitely different, mmu notifiers should be required there,
> > > afaict.  Isn't that what the current code does?
> > >
> > > See collapse_and_free_pmd() for shmem and collapse_huge_page() for anon.
> >
> > Oh, yes, of course, MADV_COLLAPSE can actually move things around and
> > properly make THPs. Thanks. But it would apply if we were only
> > collapsing PTE-mapped THPs, I think?
>
> Yes it applies I think.  And if I'm not wrong it's also doing so. :) See
> collapse_pte_mapped_thp().
>
> While for anon we always allocate a new page, hence not applicable.
>
> --
> Peter Xu

Thanks Peter!
- James