linux-kernel - Re: [PATCH 2/2] mm: add docs for per-order mTHP split counters

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Message-ID: <CAK1f24m+k+UpRnq8hP6u4uyLOkGZMt8MtX+k3fS=OTDqdA85uw@mail.gmail.com>
Date: Thu, 25 Apr 2024 13:26:11 +0800
From: Lance Yang <ioworker0@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: akpm@...ux-foundation.org, 21cnbao@...il.com, david@...hat.com, 
	baolin.wang@...ux.alibaba.com, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH 2/2] mm: add docs for per-order mTHP split counters

Hey Ryan,

Thanks for taking time to review!

On Wed, Apr 24, 2024 at 11:34 PM Ryan Roberts <ryan.roberts@....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@...il.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
>