Date: Wed, 22 May 2024 20:11:33 +0800
From: Lance Yang <ioworker0@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Barry Song <21cnbao@...il.com>, David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org, 
	willy@...radead.org, ying.huang@...el.com, ryan.roberts@....com, 
	ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: drop the 'anon_' prefix for swap-out mTHP counters

Hi Baolin,

On Wed, May 22, 2024 at 7:24 PM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
>
>
> On 2024/5/22 18:40, Barry Song wrote:
> > On Wed, May 22, 2024 at 9:38 PM Baolin Wang
> > <baolin.wang@...ux.alibaba.com> wrote:
> >>
> >>
> >>
> >> On 2024/5/22 16:58, David Hildenbrand wrote:
> >>> On 22.05.24 10:51, Baolin Wang wrote:
> >>>> The mTHP swap related counters: 'anon_swpout' and
> >>>> 'anon_swpout_fallback' are
> >>>> confusing with an 'anon_' prefix, since the shmem can swap out
> >>>> non-anonymous
> >>>> pages. So drop the 'anon_' prefix to keep consistent with the old swap
> >>>> counter
> >>>> names.
> >>>>
> >>>> Suggested-by: "Huang, Ying" <ying.huang@...el.com>
> >>>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> >>>> ---
> >>>
> >>> Am I daydreaming or did we add the anon_ for a reason and discussed the
> >>> interaction with shmem? At least I remember some discussion around that.
> >>
> >> Do you mean the shmem mTHP allocation counters in previous
> >> discussion[1]? But for 'anon_swpout' and 'anon_swpout_fallback', I can
> >> not find previous discussions that provided a reason for adding the
> >> ‘anon_’ prefix. Barry, any comments? Thanks.
> >
> > HI Baolin,
> > We had tons of emails discussing about namin and I found this email,
> >
> > https://lore.kernel.org/all/bca6d142-15fd-4af5-9f71-821f891e8305@redhatcom/
> >
> > David had this comment,
> > "I'm wondering if these should be ANON specific for now. We might want to
> > add others (shmem, file) in the future."
> >
> > This is likely how the 'anon_' prefix started being added, although it
> > wasn't specifically
> > targeting swapout.
>
> That's what I missed before. Thanks Barry.
>
> > I sense your patch slightly alters the behavior of thp_swpout_fallback
> > in /proc/vmstat.
> > Previously, we didn't classify them as THP_SWPOUT_FALLBACK, even though we
> > always split them.
>
> Sorry I did not get you here. I just re-name the mTHP swpout_fallback,
> how can this patch change the THP_SWPOUT_FALLBACK statistic counted by
> count_vm_event()?

Currently, PMD-mapped shmem folios are not accounted for in
THP_SWPOUT and related counters.

So, IMO, if we intend to account for them in those counters in the
future, removing
the 'anon_' prefix from the mTHP swap counters would be reasonable :)

Thanks,
Lance

>
> >                  if (folio_test_anon(folio) && folio_test_swapbacked(folio)) {
> >                          ...
> >                                  if (!add_to_swap(folio)) {
> >                                          int __maybe_unused order =
> > folio_order(folio);
> >
> >                                          if (!folio_test_large(folio))
> >                                                  goto activate_locked_split;
> >                                          /* Fallback to swap normal pages */
> >                                          if (split_folio_to_list(folio,
> > folio_list))
> >                                                  goto activate_locked;
> > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >                                          if (nr_pages >= HPAGE_PMD_NR) {
> >                                                  count_memcg_folio_events(folio,
> >                                                          THP_SWPOUT_FALLBACK, 1);
> >
> > count_vm_event(THP_SWPOUT_FALLBACK);
> >                                          }
> >                                          count_mthp_stat(order,
> > MTHP_STAT_ANON_SWPOUT_FALLBACK);
> > #endif
> >                                          if (!add_to_swap(folio))
> >                                                  goto activate_locked_split;
> >                                  }
> >                          }
> >                  } else if (folio_test_swapbacked(folio) &&
> >                             folio_test_large(folio)) {
> >                          /* Split shmem folio */
> >                          if (split_folio_to_list(folio, folio_list))
> >                                  goto keep_locked;
> >                  }
> >
> >
> >
> > If the goal is to incorporate pmd-mapped shmem under thp_swpout* in
> > /proc/vmstat,
> > and if there is consistency between /proc/vmstat and sys regarding
> > their definitions,
> > then I have no objection to this patch.
>
> I think this is the goal, moreover shmem will support large folio (not
> only THP) in future, so swpout related counters should be defined as
> clear as possible.
>
> However, shmem_swpout and shmem_swpout_*
> > appear more intuitive, given that thp_swpout_* in /proc/vmstat has
> > never shown any
> > increments for shmem until now - we have been always splitting shmem in vmscan.
>
> This is somewhat similar to our previous discussion on the naming of the
> shmem's mTHP counter[1], as David suggested, we should keep counter name
> consistency for now and add more in the future as needed.
>
> [1]
> https://lore.kernel.org/all/ce6be451-7c5a-402f-8340-be40699829c2@redhat.com/
> >
> > By the way, if this patch is accepted, it must be included in version
> > 6.10 to maintain
> > ABI compatibility. Additionally, documentation must be updated accordingly.
>
> Sure. I missed update the documentation, and will do in next version.
>

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