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From: Yuanchu Xie <yuanchu@google.com>
Date: Tue, 9 Apr 2024 15:36:04 -0700
Message-ID: <CAJj2-QEczZzon8AhO32_B=D2MAZG+1YWp0yrgSKQOChjQnN1OA@mail.gmail.com>
Subject: Re: [RFC PATCH v3 1/8] mm: multi-gen LRU: ignore non-leaf pmd_young
 for force_scan=true
To: "Huang, Ying" <ying.huang@intel.com>
Cc: David Hildenbrand <david@redhat.com>, "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>, 
	Khalid Aziz <khalid.aziz@oracle.com>, Henry Huang <henry.hj@antgroup.com>, 
	Yu Zhao <yuzhao@google.com>, Dan Williams <dan.j.williams@intel.com>, 
	Gregory Price <gregory.price@memverge.com>, Wei Xu <weixugc@google.com>, 
	David Rientjes <rientjes@google.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, 
	Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>, 
	Roman Gushchin <roman.gushchin@linux.dev>, Muchun Song <muchun.song@linux.dev>, 
	Shuah Khan <shuah@kernel.org>, Yosry Ahmed <yosryahmed@google.com>, 
	Matthew Wilcox <willy@infradead.org>, Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>, 
	Kairui Song <kasong@tencent.com>, "Michael S. Tsirkin" <mst@redhat.com>, 
	Vasily Averin <vasily.averin@linux.dev>, Nhat Pham <nphamcs@gmail.com>, 
	Miaohe Lin <linmiaohe@huawei.com>, Qi Zheng <zhengqi.arch@bytedance.com>, 
	Abel Wu <wuyun.abel@bytedance.com>, "Vishal Moola (Oracle)" <vishal.moola@gmail.com>, 
	Kefeng Wang <wangkefeng.wang@huawei.com>, linux-kernel@vger.kernel.org, 
	linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kselftest@vger.kernel.org
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On Mon, Apr 8, 2024 at 11:52=E2=80=AFPM Huang, Ying <ying.huang@intel.com> =
wrote:
>
> Yuanchu Xie <yuanchu@google.com> writes:
>
> > When non-leaf pmd accessed bits are available, MGLRU page table walks
> > can clear the accessed bit and promptly ignore the accessed bit on the
> > pte because it's on a different node, so the walk does not update the
> > generation of said page. When the next scan comes around on the right
> > node, the non-leaf pmd accessed bit might remain cleared and the pte
> > accessed bits won't be checked. While this is sufficient for
> > reclaim-driven aging, where the goal is to select a reasonably cold
> > page, the access can be missed when aging proactively for measuring the
> > working set size of a node/memcg.
> >
> > Since force_scan disables various other optimizations, we check
> > force_scan to ignore the non-leaf pmd accessed bit.
> >
> > Signed-off-by: Yuanchu Xie <yuanchu@google.com>
> > ---
> >  mm/vmscan.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 4f9c854ce6cc..1a7c7d537db6 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -3522,7 +3522,7 @@ static void walk_pmd_range(pud_t *pud, unsigned l=
ong start, unsigned long end,
> >
> >               walk->mm_stats[MM_NONLEAF_TOTAL]++;
> >
> > -             if (should_clear_pmd_young()) {
> > +             if (!walk->force_scan && should_clear_pmd_young()) {
> >                       if (!pmd_young(val))
> >                               continue;
>
> Sorry, I don't understand why we need this.  If !pmd_young(val), we
> don't need to update the generation.  If pmd_young(val), the bloom
> filter will be ignored if force_scan =3D=3D true.  Or do I miss something=
?
If !pmd_young(val), we still might need to update the generation.

The get_pfn_folio function returns NULL if the folio's nid !=3D node
under scanning,
so the pte accessed bit does not get cleared and the generation is not upda=
ted.
Now the pmd_young flag of this pmd is cleared, and if none of the
pte's are accessed
before another round of scanning occurs on the folio's node, the pmd_young =
check
fails and the pte accessed bit is skipped.

This is fine for kswapd but can introduce inaccuracies when scanning
proactively for
workingset estimation.

Thanks,
Yuanchu