Re: [RFC PATCH 00/26] hugetlb: Introduce HugeTLB high-granularity mapping

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Jun 27, 2022 at 09:27:38AM -0700, James Houghton wrote:
> On Fri, Jun 24, 2022 at 11:41 AM Mina Almasry <almasrymina@google.com> wrote:
> >
> > On Fri, Jun 24, 2022 at 10:37 AM James Houghton <jthoughton@google.com> wrote:
> > >
> > > [trimmed...]
> > > ---- Userspace API ----
> > >
> > > This patch series introduces a single way to take advantage of
> > > high-granularity mapping: via UFFDIO_CONTINUE. UFFDIO_CONTINUE allows
> > > userspace to resolve MINOR page faults on shared VMAs.
> > >
> > > To collapse a HugeTLB address range that has been mapped with several
> > > UFFDIO_CONTINUE operations, userspace can issue MADV_COLLAPSE. We expect
> > > userspace to know when all pages (that they care about) have been fetched.
> > >
> >
> > Thanks James! Cover letter looks good. A few questions:
> >
> > Why not have the kernel collapse the hugepage once all the 4K pages
> > have been fetched automatically? It would remove the need for a new
> > userspace API, and AFACT there aren't really any cases where it is
> > beneficial to have a hugepage sharded into 4K mappings when those
> > mappings can be collapsed.
> 
> The reason that we don't automatically collapse mappings is because it
> would take additional complexity, and it is less flexible. Consider
> the case of 1G pages on x86: currently, userspace can collapse the
> whole page when it's all ready, but they can also choose to collapse a
> 2M piece of it. On architectures with more supported hugepage sizes
> (e.g., arm64), userspace has even more possibilities for when to
> collapse. This likely further complicates a potential
> automatic-collapse solution. Userspace may also want to collapse the
> mapping for an entire hugepage without completely mapping the hugepage
> first (this would also be possible by issuing UFFDIO_CONTINUE on all
> the holes, though).
> 
> >
> > > ---- HugeTLB Changes ----
> > >
> > > - Mapcount
> > > The way mapcount is handled is different from the way that it was handled
> > > before. If the PUD for a hugepage is not none, a hugepage's mapcount will
> > > be increased. This scheme means that, for hugepages that aren't mapped at
> > > high granularity, their mapcounts will remain the same as what they would
> > > have been pre-HGM.
> > >
> >
> > Sorry, I didn't quite follow this. It says mapcount is handled

+1

> > differently, but the same if the page is not mapped at high
> > granularity. Can you elaborate on how the mapcount handling will be
> > different when the page is mapped at high granularity?
> 
> I guess I didn't phrase this very well. For the sake of simplicity,
> consider 1G pages on x86, typically mapped with leaf-level PUDs.
> Previously, there were two possibilities for how a hugepage was
> mapped, either it was (1) completely mapped (PUD is present and a
> leaf), or (2) it wasn't mapped (PUD is none). Now we have a third
> case, where the PUD is not none but also not a leaf (this usually
> means that the page is partially mapped). We handle this case as if
> the whole page was mapped. That is, if we partially map a hugepage
> that was previously unmapped (making the PUD point to PMDs), we
> increment its mapcount, and if we completely unmap a partially mapped
> hugepage (making the PUD none), we decrement its mapcount. If we
> collapse a non-leaf PUD to a leaf PUD, we don't change mapcount.
> 
> It is possible for a PUD to be present and not a leaf (mapcount has
> been incremented) but for the page to still be unmapped: if the PMDs
> (or PTEs) underneath are all none. This case is atypical, and as of
> this RFC (without bestowing MADV_DONTNEED with HGM flexibility), I
> think it would be very difficult to get this to happen.
> 

It is a good explanation. I think it is better to go to cover letter.

Thanks.

> >
> > > - Page table walking and manipulation
> > > A new function, hugetlb_walk_to, handles walking HugeTLB page tables for
> > > high-granularity mappings. Eventually, it's possible to merge
> > > hugetlb_walk_to with huge_pte_offset and huge_pte_alloc.
> > >
> > > We keep track of HugeTLB page table entries with a new struct, hugetlb_pte.
> > > This is because we generally need to know the "size" of a PTE (previously
> > > always just huge_page_size(hstate)).
> > >
> > > For every page table manipulation function that has a huge version (e.g.
> > > huge_ptep_get and ptep_get), there is a wrapper for it (e.g.
> > > hugetlb_ptep_get).  The correct version is used depending on if a HugeTLB
> > > PTE really is "huge".
> > >
> > > - Synchronization
> > > For existing bits of HugeTLB, synchronization is unchanged. For splitting
> > > and collapsing HugeTLB PTEs, we require that the i_mmap_rw_sem is held for
> > > writing, and for doing high-granularity page table walks, we require it to
> > > be held for reading.
> > >
> > > ---- Limitations & Future Changes ----
> > >
> > > This patch series only implements high-granularity mapping for VM_SHARED
> > > VMAs.  I intend to implement enough HGM to support 4K unmapping for memory
> > > failure recovery for both shared and private mappings.
> > >
> > > The memory failure use case poses its own challenges that can be
> > > addressed, but I will do so in a separate RFC.
> > >
> > > Performance has not been heavily scrutinized with this patch series. There
> > > are places where lock contention can significantly reduce performance. This
> > > will be addressed later.
> > >
> > > The patch series, as it stands right now, is compatible with the VMEMMAP
> > > page struct optimization[3], as we do not need to modify data contained
> > > in the subpage page structs.
> > >
> > > Other omissions:
> > >  - Compatibility with userfaultfd write-protect (will be included in v1).
> > >  - Support for mremap() (will be included in v1). This looks a lot like
> > >    the support we have for fork().
> > >  - Documentation changes (will be included in v1).
> > >  - Completely ignores PMD sharing and hugepage migration (will be included
> > >    in v1).
> > >  - Implementations for architectures that don't use GENERAL_HUGETLB other
> > >    than arm64.
> > >
> > > ---- Patch Breakdown ----
> > >
> > > Patch 1     - Preliminary changes
> > > Patch 2-10  - HugeTLB HGM core changes
> > > Patch 11-13 - HugeTLB HGM page table walking functionality
> > > Patch 14-19 - HugeTLB HGM compatibility with other bits
> > > Patch 20-23 - Userfaultfd and collapse changes
> > > Patch 24-26 - arm64 support and selftests
> > >
> > > [1] This used to be called HugeTLB double mapping, a bad and confusing
> > >     name. "High-granularity mapping" is not a great name either. I am open
> > >     to better names.
> >
> > I would drop 1 extra word and do "granular mapping", as in the mapping
> > is more granular than what it normally is (2MB/1G, etc).
> 
> Noted. :)
>