Re: [PATCH v5 2/4] mm: document (m)THP defer usage

[Nico Pache <npache@redhat.com>]

On Wed, Apr 30, 2025 at 2:15 PM Zi Yan <ziy@nvidia.com> wrote:
>
> On 28 Apr 2025, at 14:29, Nico Pache wrote:
>
> > The new defer option for (m)THPs allows for a more conservative
> > approach to (m)THPs. Document its usage in the transhuge admin-guide.
> >
> > Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
> >  Documentation/admin-guide/mm/transhuge.rst | 31 ++++++++++++++++------
> >  1 file changed, 23 insertions(+), 8 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> > index 5c63fe51b3ad..c50253357793 100644
> > --- a/Documentation/admin-guide/mm/transhuge.rst
> > +++ b/Documentation/admin-guide/mm/transhuge.rst
> > @@ -88,8 +88,9 @@ In certain cases when hugepages are enabled system wide, application
> >  may end up allocating more memory resources. An application may mmap a
> >  large region but only touch 1 byte of it, in that case a 2M page might
> >  be allocated instead of a 4k page for no good. This is why it's
> > -possible to disable hugepages system-wide and to only have them inside
> > -MADV_HUGEPAGE madvise regions.
> > +possible to disable hugepages system-wide, only have them inside
> > +MADV_HUGEPAGE madvise regions, or defer them away from the page fault
> > +handler to khugepaged.
> >
> >  Embedded systems should enable hugepages only inside madvise regions
> >  to eliminate any risk of wasting any precious byte of memory and to
> > @@ -99,6 +100,15 @@ Applications that gets a lot of benefit from hugepages and that don't
> >  risk to lose memory by using hugepages, should use
> >  madvise(MADV_HUGEPAGE) on their critical mmapped regions.
> >
> > +Applications that would like to benefit from THPs but would still like a
> > +more memory conservative approach can choose 'defer'. This avoids
> > +inserting THPs at the page fault handler unless they are MADV_HUGEPAGE.
> > +Khugepaged will then scan the mappings for potential collapses into (m)THP
>
> How about the text below? It explicitly states khugepaged behavior.
>
> Khugepaged will then scan all mappings, even those not explicitly marked
> with MADV_HUGEPAGE, for potential collapses into (m)THPs.

I agree, this reads better. I can modify it on the V6 :)
>
> > +pages. Admins using this the 'defer' setting should consider
> > +tweaking khugepaged/max_ptes_none. The current default of 511 may
> > +aggressively collapse your PTEs into PMDs. Lower this value to conserve
> > +more memory (i.e., max_ptes_none=64).
> > +
> >  .. _thp_sysfs:
> >
> >  sysfs
> > @@ -109,11 +119,14 @@ Global THP controls
> >
> >  Transparent Hugepage Support for anonymous memory can be entirely disabled
> >  (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE
> > -regions (to avoid the risk of consuming more memory resources) or enabled
> > -system wide. This can be achieved per-supported-THP-size with one of::
> > +regions (to avoid the risk of consuming more memory resources), deferred to
> > +khugepaged, or enabled system wide.
> > +
> > +This can be achieved per-supported-THP-size with one of::
> >
> >       echo always >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
> >       echo madvise >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
> > +     echo defer >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
> >       echo never >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
> >
> >  where <size> is the hugepage size being addressed, the available sizes
> > @@ -136,6 +149,7 @@ The top-level setting (for use with "inherit") can be set by issuing
> >  one of the following commands::
> >
> >       echo always >/sys/kernel/mm/transparent_hugepage/enabled
> > +     echo defer >/sys/kernel/mm/transparent_hugepage/enabled
> >       echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
> >       echo never >/sys/kernel/mm/transparent_hugepage/enabled
> >
> > @@ -286,7 +300,8 @@ of small pages into one large page::
> >  A higher value leads to use additional memory for programs.
> >  A lower value leads to gain less thp performance. Value of
> >  max_ptes_none can waste cpu time very little, you can
> > -ignore it.
> > +ignore it. Consider lowering this value when using
> > +``transparent_hugepage=defer``
> >
> >  ``max_ptes_swap`` specifies how many pages can be brought in from
> >  swap when collapsing a group of pages into a transparent huge page::
> > @@ -311,14 +326,14 @@ Boot parameters
> >
> >  You can change the sysfs boot time default for the top-level "enabled"
> >  control by passing the parameter ``transparent_hugepage=always`` or
> > -``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` to the
> > -kernel command line.
> > +``transparent_hugepage=madvise`` or ``transparent_hugepage=defer`` or
> > +``transparent_hugepage=never`` to the kernel command line.
> >
> >  Alternatively, each supported anonymous THP size can be controlled by
> >  passing ``thp_anon=<size>[KMG],<size>[KMG]:<state>;<size>[KMG]-<size>[KMG]:<state>``,
> >  where ``<size>`` is the THP size (must be a power of 2 of PAGE_SIZE and
> >  supported anonymous THP)  and ``<state>`` is one of ``always``, ``madvise``,
> > -``never`` or ``inherit``.
> > +``defer``, ``never`` or ``inherit``.
> >
> >  For example, the following will set 16K, 32K, 64K THP to ``always``,
> >  set 128K, 512K to ``inherit``, set 256K to ``madvise`` and 1M, 2M
>
> Otherwise, LGTM. Thanks. Reviewed-by: Zi Yan <ziy@nvidia.com>
>
> --
> Best Regards,
> Yan, Zi
>