This commit introduces documentation for mTHP split counters in
transhuge.rst.

Signed-off-by: Lance Yang <ioworker0@gmail.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index f82300b9193f..35d574a531c8 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -475,6 +475,22 @@ anon_swpout_fallback
 	Usually because failed to allocate some continuous swap space
 	for the huge page.
 
+split_page
+	is incremented every time a huge page is split into base
+	pages. This can happen for a variety of reasons but a common
+	reason is that a huge page is old and is being reclaimed.
+	This action implies splitting all PMD/PTE mapped with the huge page.
+
+split_page_failed
+	is incremented if kernel fails to split huge
+	page. This can happen if the page was pinned by somebody.
+
+deferred_split_page
+	is incremented when a huge page is put onto split
+	queue. This happens when a huge page is partially unmapped and
+	splitting it would free up some memory. Pages on split queue are
+	going to be split under memory pressure.
+
 As the system ages, allocating huge pages may be expensive as the
 system uses memory compaction to copy data around memory to free a
 huge page for use. There are some counters in ``/proc/vmstat`` to help
-- 
2.33.1

On 24/04/2024 14:51, Lance Yang wrote:
> This commit introduces documentation for mTHP split counters in
> transhuge.rst.
> 
> Signed-off-by: Lance Yang <ioworker0@gmail.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index f82300b9193f..35d574a531c8 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -475,6 +475,22 @@ anon_swpout_fallback
>  	Usually because failed to allocate some continuous swap space
>  	for the huge page.
>  
> +split_page
> +	is incremented every time a huge page is split into base

perhaps "...successfully split into base..." to make it clear that this is only
incremented on success.

> +	pages. This can happen for a variety of reasons but a common
> +	reason is that a huge page is old and is being reclaimed.
> +	This action implies splitting all PMD/PTE mapped with the huge page.

What does it mean to "split all PTE"? It's already at its smallest granularity.
Perhaps "This action implies splitting any block mappings into PTEs."?

> +
> +split_page_failed
> +	is incremented if kernel fails to split huge
> +	page. This can happen if the page was pinned by somebody.
> +
> +deferred_split_page
> +	is incremented when a huge page is put onto split
> +	queue. This happens when a huge page is partially unmapped and
> +	splitting it would free up some memory. Pages on split queue are
> +	going to be split under memory pressure.
> +
>  As the system ages, allocating huge pages may be expensive as the
>  system uses memory compaction to copy data around memory to free a
>  huge page for use. There are some counters in ``/proc/vmstat`` to help

Hey Ryan,

Thanks for taking time to review!

On Wed, Apr 24, 2024 at 11:34 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 24/04/2024 14:51, Lance Yang wrote:
> > This commit introduces documentation for mTHP split counters in
> > transhuge.rst.
> >
> > Signed-off-by: Lance Yang <ioworker0@gmail.com>
> > ---
> >  Documentation/admin-guide/mm/transhuge.rst | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> > index f82300b9193f..35d574a531c8 100644
> > --- a/Documentation/admin-guide/mm/transhuge.rst
> > +++ b/Documentation/admin-guide/mm/transhuge.rst
> > @@ -475,6 +475,22 @@ anon_swpout_fallback
> >       Usually because failed to allocate some continuous swap space
> >       for the huge page.
> >
> > +split_page
> > +     is incremented every time a huge page is split into base
>
> perhaps "...successfully split into base..." to make it clear that this is only
> incremented on success.

Yep. Your suggestion does make it clearer.

>
> > +     pages. This can happen for a variety of reasons but a common
> > +     reason is that a huge page is old and is being reclaimed.
> > +     This action implies splitting all PMD/PTE mapped with the huge page.
>
> What does it mean to "split all PTE"? It's already at its smallest granularity.
> Perhaps "This action implies splitting any block mappings into PTEs."?

Nice. It would be clearer and better!

Thanks again for the suggestions!
Lance

>
> > +
> > +split_page_failed
> > +     is incremented if kernel fails to split huge
> > +     page. This can happen if the page was pinned by somebody.
> > +
> > +deferred_split_page
> > +     is incremented when a huge page is put onto split
> > +     queue. This happens when a huge page is partially unmapped and
> > +     splitting it would free up some memory. Pages on split queue are
> > +     going to be split under memory pressure.
> > +
> >  As the system ages, allocating huge pages may be expensive as the
> >  system uses memory compaction to copy data around memory to free a
> >  huge page for use. There are some counters in ``/proc/vmstat`` to help
>