Re: [PATCH 09/46] mm: add MADV_SPLIT to enable HugeTLB HGM

["Zach O'Keefe" <zokeefe@google.com>]

On Thu, Jan 5, 2023 at 7:29 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 05.01.23 11:18, James Houghton wrote:
> > Issuing ioctl(MADV_SPLIT) on a HugeTLB address range will enable
> > HugeTLB HGM. MADV_SPLIT was chosen for the name so that this API can be
> > applied to non-HugeTLB memory in the future, if such an application is
> > to arise.
> >
> > MADV_SPLIT provides several API changes for some syscalls on HugeTLB
> > address ranges:
> > 1. UFFDIO_CONTINUE is allowed for MAP_SHARED VMAs at PAGE_SIZE
> >     alignment.
> > 2. read()ing a page fault event from a userfaultfd will yield a
> >     PAGE_SIZE-rounded address, instead of a huge-page-size-rounded
> >     address (unless UFFD_FEATURE_EXACT_ADDRESS is used).
> >
> > There is no way to disable the API changes that come with issuing
> > MADV_SPLIT. MADV_COLLAPSE can be used to collapse high-granularity page
> > table mappings that come from the extended functionality that comes with
> > using MADV_SPLIT.
> >
> > For post-copy live migration, the expected use-case is:
> > 1. mmap(MAP_SHARED, some_fd) primary mapping
> > 2. mmap(MAP_SHARED, some_fd) alias mapping
> > 3. MADV_SPLIT the primary mapping
> > 4. UFFDIO_REGISTER/etc. the primary mapping
> > 5. Copy memory contents into alias mapping and UFFDIO_CONTINUE the
> >     corresponding PAGE_SIZE sections in the primary mapping.
> >
> > More API changes may be added in the future.
> >
> > Signed-off-by: James Houghton <jthoughton@google.com>
> > ---
> >   arch/alpha/include/uapi/asm/mman.h     |  2 ++
> >   arch/mips/include/uapi/asm/mman.h      |  2 ++
> >   arch/parisc/include/uapi/asm/mman.h    |  2 ++
> >   arch/xtensa/include/uapi/asm/mman.h    |  2 ++
> >   include/linux/hugetlb.h                |  2 ++
> >   include/uapi/asm-generic/mman-common.h |  2 ++
> >   mm/hugetlb.c                           |  3 +--
> >   mm/madvise.c                           | 26 ++++++++++++++++++++++++++
> >   8 files changed, 39 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> > index 763929e814e9..7a26f3648b90 100644
> > --- a/arch/alpha/include/uapi/asm/mman.h
> > +++ b/arch/alpha/include/uapi/asm/mman.h
> > @@ -78,6 +78,8 @@
> >
> >   #define MADV_COLLAPSE       25              /* Synchronous hugepage collapse */
> >
> > +#define MADV_SPLIT   26              /* Enable hugepage high-granularity APIs */
>
> I think we should make a split more generic, such that it also splits
> (pte-maps) a THP. Has that been discussed?


Thanks James / David.

MADV_SPLIT for THP has come up a few times; firstly, during the
initial RFC about hugepage collapse in process context, as the natural
inverse operation required by a generic userspace-managed hugepage
daemon, the second -- which is more immediately practical -- is to
avoid stranding THPs on the deferred split queue (and thus still
incurring the memcg charge) for too long [1].

However, its exact semantics / API have yet to be discussed / flushed
out (though I'm planning to do exactly this in the near-term).

Just as James has co-opted MADV_COLLAPSE for hugetlb, we can co-opt
MADV_SPLIT for THP, when the time comes -- which I think makes a lot
of sense.

Hopefully I can get my ducks in order to start a discussion about this
eminently.

Best,
Zach

[1] https://lore.kernel.org/linux-mm/YZ9kUD5AG6inbUEg@xz-m1.local/

> --
> Thanks,
>
> David / dhildenb
>