linux-kernel - Re: [PATCH] smaps: count large pages smaller than PMD size to anonymous

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Message-ID: <CAGsJ_4z_nQXrnjWFODhhNPW4Q0KjeF+p+bXL5D0=CxskWo1_Jg@mail.gmail.com>
Date: Sun, 8 Dec 2024 14:06:57 +0800
From: Barry Song <21cnbao@...il.com>
To: Lance Yang <ioworker0@...il.com>
Cc: David Hildenbrand <david@...hat.com>, Wenchao Hao <haowenchao22@...il.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>, 
	Oscar Salvador <osalvador@...e.de>, Muhammad Usama Anjum <usama.anjum@...labora.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Ryan Roberts <ryan.roberts@....com>, Peter Xu <peterx@...hat.com>, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH] smaps: count large pages smaller than PMD size to anonymous_thp

On Fri, Dec 6, 2024 at 7:16 PM Lance Yang <ioworker0@...il.com> wrote:
>
> On Tue, Dec 3, 2024 at 10:17 PM David Hildenbrand <david@...hat.com> wrote:
> >
> > On 03.12.24 14:49, Wenchao Hao wrote:
> > > Currently, /proc/xxx/smaps reports the size of anonymous huge pages for
> > > each VMA, but it does not include large pages smaller than PMD size.
> > >
> > > This patch adds the statistics of anonymous huge pages allocated by
> > > mTHP which is smaller than PMD size to AnonHugePages field in smaps.
> > >
> > > Signed-off-by: Wenchao Hao <haowenchao22@...il.com>
> > > ---
> > >   fs/proc/task_mmu.c | 6 ++++++
> > >   1 file changed, 6 insertions(+)
> > >
> > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > > index 38a5a3e9cba2..b655011627d8 100644
> > > --- a/fs/proc/task_mmu.c
> > > +++ b/fs/proc/task_mmu.c
> > > @@ -717,6 +717,12 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
> > >               if (!folio_test_swapbacked(folio) && !dirty &&
> > >                   !folio_test_dirty(folio))
> > >                       mss->lazyfree += size;
> > > +
> > > +             /*
> > > +              * Count large pages smaller than PMD size to anonymous_thp
> > > +              */
> > > +             if (!compound && PageHead(page) && folio_order(folio))
> > > +                     mss->anonymous_thp += folio_size(folio);
> > >       }
> > >
> > >       if (folio_test_ksm(folio))
> >
> >
> > I think we decided to leave this (and /proc/meminfo) be one of the last
> > interfaces where this is only concerned with PMD-sized ones:
> >
> > Documentation/admin-guide/mm/transhuge.rst:
> >
> > The number of PMD-sized anonymous transparent huge pages currently used by the
> > system is available by reading the AnonHugePages field in ``/proc/meminfo``.
> > To identify what applications are using PMD-sized anonymous transparent huge
> > pages, it is necessary to read ``/proc/PID/smaps`` and count the AnonHugePages
> > fields for each mapping. (Note that AnonHugePages only applies to traditional
> > PMD-sized THP for historical reasons and should have been called
> > AnonHugePmdMapped).
>
> Yeah, I think we need to keep AnonHugePages unchanged within these interfaces
> due to historical reasons ;)
>
> Perhaps, there might be another way to count all THP allocated for each process.

My point is that counting the THP allocations per process doesn't seem
as important
when compared to the overall system's status. We already have
interfaces to track
the following:

* The number of mTHPs allocated or fallback events;
* The total number of anonymous mTHP folios in the system.
* The total number of partially unmapped mTHP folios in the system.

To me, knowing the details for each process doesn’t seem particularly
critical for
profiling.  To be honest, I don't see a need for this at all, except perhaps for
debugging to verify if mTHP is present.

If feasible, we could explore converting Ryan's Python script into a native
C program. I believe this would be more than sufficient for embedded systems
and Android.

>
> Thanks,
> Lance
>
>
> >
> >
> >
> > --
> > Cheers,
> >
> > David / dhildenb

Thanks
Barry